What are some practical steps to rebuild confidence and motivation after a rough start in college?

I’m in a 3 yr long situationship where the guy doesn’t want to be exclusive but we have a strong connection and he says he loves me. I’ve started feeling guilty about things I’ve done in the past like flirting with other guys, what should I do?
Why is it that every time something good happens to myself, I self-sabotage it because I don’t think I deserve it? How do I stop this?
Can you share any specific moments where your American manners caused confusion or misunderstanding in France?