How to be more positive

My girlfriend and I broke up this morning. It was her decision. She said it was due to my general negative outlook on things without positivity or solutions being sought out. This is something I know I struggle with and unless I’m actively and solely making that the focus of my life then I always slip eventually and drop off into the negativity again without meaning to. I’d like to change this about myself to where being more positive and having a better outlook on things comes naturally rather than the opposite before trying things with her again. She has a couple things to work on on her own too but this is my problem and even if I don’t wind up getting back with her I’d really like to adjust this behavior and have a better chance at this not effecting future relationships in the same way, as she it’s the only partner to have expressed my negativity weighs on them in this way.