My girlfriend started out not unlike you. She cheated on virtually every partner she'd ever had.
Eventually she learned about poly and it changed her life. She started living openly with her partners, started being honest with herself and with them that she could not do monogamy.
So it is possible to reform.
But I think before you start working on being poly, you need to work on yourself. To be blunt: what you describe as "cowboy ways" I describe as "being a jerk." It's not enough to state that as a fact, you have to actually change it. Start doing things for other people. At first, you'll just have to force yourself, but eventually you'll learn how wonderful it feels to make someone happy (other than yourself). Once you get a taste of the reward, you'll probably start doing it more, and that can really be an upward spiral.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker