Why am I nervous all the time with my girlfriend?

It can be a number of things. You might like her so much that you just are afraid if the relationship might fail. You might also overthink things with your girlfriend. That you worry to much and forget just how to relax.

Because you are in love with her. It is normal to be around your girlfriend or boyfriend even. You just have really strong feelings for her and it makes you feel awkward and nervous about being around her. Just talk to her about it and try and connect to her. The best thing that you can do for yourself is try and think about what exactly it is about her that makes you so nervous. Don't be afraid to talk to her about stuff. She is there for you to talk to and if she really loves you she gets why you are so nervous. Just keep your head up.