To say you should start by not being negative about what you are saying, may seem obvious, yet I've seen many negative things listed nonetheless in answer to your question.
Let me give you examples, it may be simpler.
Negatives is all that says "NO", as well as all that is saddening, frightening, depressing.
You do not go and tell yourself it could be worse, when you are having a bad day; that would be bad enough on an ok day, but in one you are feeling low, telling yourself you could have a worse day than that is NEGATIVE.
Telling yourself tomorrow/someday is going to be better, is positive.
It's the old adage: the difference between people who always see the glass half full (positive people) and those who always see it half-empty (negative people).
When you think negatively, correct yourself and say " ooops, I meant [... adding the correct way of wording it out positively].
Simple tricks work too.
You stop yourself when you are starting to feel down or are having a very bad day.
You then breath out extremely long and slowly, being very aware of what you are doing. Breath out until you need and catch your breath again. Then breath in quite slowly. And breath out again very slowly for as long as you can without breathing in. Then in and out again slowly. The slower and longer you can breath out, the clearer and more positive you will be feeling. It will not be enough to make the day suddenly bright. But it will be enough for you to brake free from a pattern and some immediate stress, and start acting positively about it.
Do not ramble over what's happened today, not until you feel you are having a good day instead, then you could chance it. When you are having a very bad day or are feeling really low, you can not be sure that looking over what went wrong to find something good in it will work out the way you expected it. You are more likely to get further depressed. Instead think about good things you still have time for today, or will have time for tomorrow/at the weekend/next week/during your next holidays....
Once you are more used to catching yourself before you go too low, you can pick up what went wrong at that very moment, and instead of feeling depressed or angered about it, look positively at it and think of what you can do to better that next time such a situation arises.
Do not tell yourself you could have avoided making a mess of it by doing this or that. That's negative, even if to use it for a better go next time around. What you should be telling yourself instead is: should such a situation arise, what can I do to get it right?
Do you see the difference? You do not linger on the fact that you got something wrong (even if to say you could do it better some other time), for that is reminded your subconscious that you are capable of doing the worse for yourself. When you are having a bad day, that's the very last thing you need and hammer in, right? So instead of telling yourself that you are a failure, directly or indirectly, tell yourself you are someone who can and will handle any such situation that comes up in the future.
If you are having a really bad day, you don't want half-positive things, for your mind will already making you feel quite negative about yourself, and so a fully positive thing, will only have a mild positive balm to it, what would a half-positive one have? In psychology, it has a negative effect, and a negative thinking has a disastrous effect. So be one step ahead in positivity.
On a good day though, if something reminds you of a failure, be that truthful instead and admit that you made a mistake in dealing with something, but that this was how one learns about life, and that it means in the future you will be doing the right thing. Again do not tell yourself that in the future you will not be doing the wrong thing again. Instead tell yourself that in the future you will be doing the right thing.
It may only be words to you, but whether you are at ease with handling words or not as a person, it's the same for everyone: being told you did the right thing always sounds better than hearing you will not do the wrong thing again. For after all, you'd not be doing a disaster again then, but would you be doing the right or better thing? And the "again" part, being a constant reminder that you at least once failed! You do not want that.
You will be thinking about how to do the right thing in the future. That way you will know what will be the right thing in the future. Now you are capable of doing the right thing And so, you will do the right thing in the future.
These are the stages in you positive attitude to anything bad that has happened.
So if it's true that lots of worse things could probably have happened to you, that's not helping for it's telling you that there could be worse to come.
Think about good things that could come, think about how to handle correctly a situation that was wrong without going over the fact that it was wrong. And then imagine yourself facing up to that situation again and handling it right in the future. Then you can tell yourself that you are ready for the future, without adding whatever the past, or unlike in the past.
I'll add one little detail most people seem to forget these days and then complain that they are bored or depressed:
Whatever makes you feel you are having a bad day, you can change.
I'm not joking.
Let's say you want to meet more people or make more friends but you are thinking you are not succeeding. What about taking up a membership in a club, even one online, to join a group of people who like what you like? If you do not make friends locally, it's not for you are unlikeable but for people around you do not like what you like or are not aware that you like the same things they do. Do not fool yourself by pretending you like to play football if you prefer to play chess. Others are no fools either, they'll sense your lack of or faked interest in their hobby. It will make you sound false, and therefore not someone one would want as a friend.
If you feel bored, what about asking yourself if you've really tried and do something at all? Watching TV or playing a video game may be fun for a while, but it will never be the same as sharing something with another human being. So whether that requires being with someone playing a game or visiting some place; or whether you are taking up knitting and then have people looking at what you've done and asking you how to do this too, that's up to you.
Have you made your mind work at all today? Save the world, be creative, write up the ingredients of a perfect party (for when you will have friends, money, the room for it, the time for it, the reason for the party). And really think hard about it. What lighting would you like, what set up for it? How would you invite the people? Would you make it into a murder mystery? any ideas about what the mystery would be like?
Get some fresh air! It's raining like crazy? Who cares, you are alive, the rain is soft on your face like a cuddle or it's harsh like a tempest, either way you are feeling full of life and aware of the elements around you. It's not a depressing rain unless you let it be so. It's the rain cleaning you of you depression or it's the rain being so wild that you are getting a bit wild too instead of down. It's a grey rain showing you that after it there is a rainbow somewhere, even if you may not see it today.
Get some food. No, not for a binge, but something very fresh and raw and taste it. Taste it carefully, what does this fruit or carrot taste like? Is it crunchy and full of bite? Is it sweet or does it burn your tongue? IS it juicy? What does it smell like? How beautiful is it to look at? Now anything in your life you could get some bite, some sweetness, some vitamins into? You can do it by being more positive and more full of life, just like that raw fruit, vegetable or chili pepper you've just had a bite into.
And smile, laugh; even if you do not feel like it at first, there must be a story making you laugh, or a thought making you smile; Think about it, practice smiling and laughing. Do it at home, do it outside and if someone is asking you why you are laughing just tell them it's for you are having a great day! And do they want to hear a funny story you've just remembered?
They'll laugh and smile too, and somewhere deep down you'll know you made something positive out of today afterall.