So, I admit I am insecure. I lack relationship and sex experience, have some symptoms of nice guy syndrome, although I don't think I am one. I can be fun, but I am sensitive and introverted. It takes long time to really get to know me, but I am really worried about my insecurities eventually causing jealousy and needy behaviour. Is there a way how to prevent this?

I know jealousy is useless, but I just think that in a moment when she finds out I am not so confident how she thinks I am, it will inevitably lead to cheating/break up.

But I have made some progress. I made boundaries for myself, assert myself more, more saying no, more saying my opinions and stand for them, also started working out but that little rabbit in my head is just telling me I am not good enough and couldn't keep a girl, let alone beautiful quality one...

tl;dr: guy is insecure, worries about cheating because of insecurities, although he's making some progress, he fears he will be just needy pushover in a relationship.