Performance anxiety with girlfriend

For the past 4 months I've been dating a new girl, I really love this girl and want the best for her and our relationship. Last month we started getting hot and heavy and she decided she was ready for sex, as was I (I thought). I had an erection when we started cuddling and getting ready and when she said that she was ready I immediately lost it. She fully understands and wants to help me through this. We have tried about 3 more times after that and the last time I managed to get the condom on and have sex for about 40 seconds before losing my erection, it wasn't the most comfortable position as I was holding her up. I've been masturbating since about 11 about 5 times a week mainly to porn. I just want to be able to please this girl and have decent sex with her as I find her extremely attractive. I've considered NoFap in the past but I completely flatlined after a week and I relapsed incase another encounter of sec arouse and I wasn't ready. I was thinking about buying an L arginine supplement to support my blood vessels. I have had social anxiety in the past but got over it for the most part. I really need help as I love this girl and want to be able to have sex with her. I don't want to completely flatline and I want to try to remain sexual with her as much as I can. Any advice is useful. I'm begging for help. Sorry that this post was a huge mess.