Sit down for 5 minutes each day, preferably first thing in the morning as it'll be easiest to do it and it'll impact the rest of your day, and make it your intention to write about something from a completely positive perspective. Whether it's coming up with positive reasons for why writing positively will help you, or writing about a recent or not-so-recent accomplishment, or even literally about why the colour of your walls is so good. Even if it seems silly, do it, in fact that might help because being silly will put you in a good mood and make it easier to think more positively. Even if in your head you're coming up with reasons for why what you're writing is false, just continue doing it. Start exercising your positivity muscle.
Do this every single day for the next 6 months and I'll be very surprised if you haven't seen improvement. If you want you can also extend the time you do this for as time goes on. But start small, just do the bare minimum until it's easy, then jump up a level. This may take a long time so don't get discouraged, like I said the entire time you may be saying in your head how bullshit this is. Just keep moving forward.
Beyond that, continue meditating, eating healthy, going to the gym, etc, etc. All those things will make it far easier to think positively. Also check out Leo's video, "How to Stop Being a Victim - Part 1." It'll help a lot.
