I believe the first step would be just finding positive things to put in my head as a way to force out the negative nonsense that I’ve been filling it up with.
That's a darn good way to start. The book GreenChair was referring to is "Codependent No More", perhaps the one book that helped many of us most. It is well written in language clearly understood and the first time I read it, I wanted royalties, because it was as if she was writing ab out me and my dysfunctional life. Libraries carry it as well as book stores, maybe get a copy for yourself and see if it doesn't help you like it helped many of us.
Meetings helped me a lot too, CoDA (Codependents Anonymous) was my home group and I occasionally went to Al-anon as well. These fellowships are about US and our recovery and not about our addicted loved ones at all. It helped me understand why I was the way I was and gave me a good path to take to heal and move on to a healthier, happier life.
If your depression continues it may be good to see a counselor or your doctor as well. It could be more than the scars of bad relationships and there is no shame in asking for help.
Good luck, keep posting and let us know how you are doing.