I don't cheer myself up. If I'm having a bad day (call it a "downer", whatever) I just realise that it's not a permanent state/feeling/aspect and that it will pass.
It's a process of understanding your state of mind and accepting the now and leaving time and change to do what is necessary.
So much this. The big thing is being aware of your mind, it seems you've got that part covered already.

Next up:
1) Accept occasionally the mind can slip. That itself is no big deal.
2) Accept that that is just your brain, an awesome but also fairly dumb fleshy organ, not
you.
3) Keep a mental whip ready. Whenever you sense your dumb fleshy brain starts slipping into a pool of despair and whiny self-pity,
whip it. Say:"I am not falling for your crap, dumb mushy brain. Get back to work."
4) The sooner you whip your brain, the better it works. The worse you are at detecting depression setting in, the later you whip, the less it works.
5) This will work for a little bit, but within minutes your dumb brain will give it another go.
Whip it.
6) This is a matter of practice. Initially this will be very very hard. The more you practice, the better you'll get at it.
7) In case you are wondering: there is ample and very interesting scientific literature backing this up. You can experimentally induce all kinds of negative physical reactions in your body (heart rate, skin conductance, respiratory patterns) and brain. Experiments have shown over and over you can arm yourself against this. Even to the point that it becomes very hard, or with experiments that are limited by ethical restrictions,
impossible to induce these basic physiological responses to people trained in various ways.
8) The issue is we, especially in our culture, are not trained or schooled in this at all. We tend to be slaves to our brains, rather than having our brains be a tool we control.
Finally, and an important but hopefully needless thing: if the above fails, which it might especially in the beginning, and you feel you are losing control of your mind in a bad way,
call for help. Our brains may be faulty and daft, but they have tens of thousands of years of experience in making us feel like crap for no good reason. Dont feel bad about asking for help now and again.