[22M][23F] How to maintain an erection during intercourse?

Hi there,

TL;DR
I don't have much experience with women, I can count on one hand for the amount of times I did it with someone. Though since more than a month ago I fell in love with this this girl and crossed the line only recently. I consumed a lot of porn in the past, still watch porn nowadays but way less frequent.

During our time together I actively avoided anything involving my penis and try to please her instead. Which worked but when it finally happened I could not maintain an erection with her, and it kind went soft while she was busy. So instead we started kissing while I did it myself which resulted in me finishing after 20-30 seconds.

Later after alot of foreplay we tried again and it went a lot better, though after about a minute it happened again and tried playing it off. It's not like I cant get an erection cause I do get them all the through various situations with her but when it's actually "needed" it does nothing unless I look away and think of something else.

This was already a thing with past girls I've been with, though I was brushing it off cause I may not have liked them enough. But now facing reality Im not sure what to do. Ive been actively working out for months, and my diet is very strict now.

Any advice to improve myself on this?