I just saw a different post about something similar so i thought

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I just saw a different post about something similar so i thought id post my problem too, /adv/.

Ive always been a romantic kind of dude, even when i was a little kid.
Course, I was sorta really beta and would get my hopes up for a girl who i was infatuated with only to be let down big time.
I finally met a girl at age 16 who i fell into a relationship with. We had obvious problems but i wanted to have someone so much that i overlooked them. I went all out for this girl, and she knew it and took advantage of it. When the relationship died a year later, i was really bitter because of how much trust i had put into a girl, only for her to cheat on me and tell me that it doesnt matter how hard i try to make her happy, she wont love me.
Fast forward 6 months, im over her but still a bit bitter. I meet this girl that is by far everything i could ever want in a girl, and then some. Looks, personality, interests, the whole deal. Compatible in every way, and what i had always wanted but never thought id ever get.
Ive been with her for 3 years and i just cant get myself to truly love her with a passion like she does to me. Ive cheated on her (never in person, just sent nudes and what not) and there are times where i feel nothing towards her at all. I know its stupid, and whats keeping me to her is the fact that i know im going to regret ever losing her if i do. Without making excuses for myself, i know im a piece of shit for what im doing but why is it so hard for me to be passionate about someone i never ever thought i could ever get? Am i still bitter/jaded from the years of being let down and the failed relationship i had before?