You sound a lot like me up until a few years ago. And if you’re anything like me, being painfully shy is actually massive low self-esteem. It was only when I started “teaching” myself that I was every bit as worthy as anyone else out there, by telling myself over and over again, every day, that I am just as good as the next person, that my opinion is worth just as much as the next person’s, etc. that I started to become more comfortable with people. Speaking out in groups is still daunting, but I don’t avoid it anymore. Still working on it!!
Having said all that, your girlfriend needs to know and understand who you are, and not take it personally when you don’t “perform” according to her expectations… You’re not making any mistakes, you’re just being you, and you have every right to be you, every step of the way from the present you to the future you.