Welcome to the boards.
I can't give you an easy answer to why you feel the way you do - our emotions are complicated things - but we can talk about them a bit and see if we can understand them a bit more. When you say you feel guilt, what do you think you feel guilty about? Same with the anxiety - what worries you?
You say you've spent a lot of time lying in bed, overthinking and panicking. Have you reached out to anyone for help with this? What are you doing to look after yourself? I'm wondering if this is about the sex, or if it's something more general that some self-care might help with. A good place to start might be getting up and trying to do things, even simple ones like going for a walk, or calling a friend, or drawing something. There are more ideas here Self-Care a La Carte
. If you can move out of that panicked space, it might give you room to think about what your real thoughts and feelings are about sex and your relationship. What do you think?
Whilst you're figuring those things out, maybe it would be a good idea to hold off on the sex for a little bit? Spend time with your girlfriend doing other things?