how to get over sexual anxiety?

me (18m) and my gf (19f) get very intimate a lot. we are both young and inexperienced in sexual activities but we are comfortable with each other and always try stuff. we haven’t attempted sex yet, as this would be both of our first time. we’ve done other stuff such as she’s attempted to give me a handjob but everytime we try it, i lose my erection because i keep thinking that i’m not gonna put on a good performance for her, like as if she wouldn’t enjoy it. the first time we tried was the first time we’ve done anything sexual, so i was nervous she wouldn’t like my penis or it wouldn’t be good enough and those thoughts caused anxiety and caused me to lose my erection, she thought that it was because she wasn’t doing it right but i explained to her what happened and she’s very understanding and knows what i’m going through. we’ve tried doing handjobs more and even oral sex but everytime we try i lose my erection because of my fear of not being able to keep my erection. i know it’s not any problem health wise because when we kiss and touch each other i have an erection, it’s when the act happens i try not to get that fear of not finishing or performing. does anyone have any tips with this and how can i deal with this with my gf? i know she wants to do these things with me and i keep messing it up which adds to me fear. please help