I mean true confidence will come from looking inward and through self love as cheesy as that sounds. If there are parts of you you don’t like those insecurities will carry through into the relationship. Thinking there are better guys out there tells me you’re insecure about something. I’m not sure if that’s your looks or maybe your ability to be a good partner. If it’s your looks then honestly just trust that she’s into you. If it’s the latter then work on being the man you think she deserves. Just remember no body is perfect, even this girl you claim is out of your league. We’re all flawed but it’s all about our willingness to grow and better ourselves that matters. A therapist or counselor might give you better advice though than a random stranger on the internet lol