~speaking from a little bit of experience~
sounds like u might have a pretty nicely developed case of performance anxiety / erectile dysfunction.
had an experience where u were maybe nervous about the fact that u were into this girl? wanted to really show her a good time, couldn't get it up, felt inadequate, frustrated, etc
experience repeats itself, further stressing u out and cementing the possibility that this will continue to happen
over time [could be days - years], sex stops being something exciting, fun, relaxing, pleasurable, etc., and starts being a stressful test: will i get it up? will this be fun for anyone? what is she gonna think when the magic inevitably doesn't happen?
u become increasingly conditioned to go into a stressful, fight-or-flight mindset rather than an aroused, excited one
sexual arousal & libido dry up, completing the cycle of disinterest and performance anxiety
couple of things you can do! first, check and make sure it's a mental thing and not a medical issue. the fact that ur still masturbating and getting hard suggests that the plumbing works, but i'm not a doctor. usually doctors will have u check and make sure ur having erections while u sleep by having u put a bit of tape or equally flimsy substitute around ur flaccid penis at night, and in the morning checking to see if the tape is broken. if it is, all good. also hormone levels and lifestyle (i.e. smoking, drinking, exercise, sleeping, amount of other stressors in ur life, etc.) are big contributing factors, and a doctor can do a hormone test if ur worried it might be low testosterone.
if it's psychological, there are various ways to 'retrain' ur brain to associate arousal with a positive, excited, and horny state - a quick internet search of "male sexual performance anxiety" will turn up some good resources. watching less porn – particularly male-orgasm-oriented scenes – as well as masturbating less can help. exercise is always good, and the supplement l-arganine (promotes circulation and heart / blood vessel health) is sometimes coupled with exercise to treat ED; worst case scenario, it doesn't really help physiologically, but it's a super safe amino acid and could still be beneficial, even if only for the placebo effect.
it also will probably be really helpful to talk about this – with a partner u trust, friends, and especially with a sex psychologist / counselor – and get as comfortable with it as possible . reducing the stigma of 'holyshit impotence!!!' is key. additionally, it may be that ur gonna have to confront some deeper issues with ur attitude towards sex. do u feel responsible for her orgasm? do u feel like if u don't cum, the sex was unsuccessful? the first one in particular really fked me up when i was younger, and to some extent still does, and i'm here to tell u that, while it's a pain to reframe ur sexual perspective / identity, it's really worth it in the long run.
tl;dr - sex shouldn't be stressful or scary! rule out a physical issue, focus on possible underlying psychological causes, do some research, talk it out with people u trust (and optimally a sexually partner), & feel better & less stigmatized!! alright!! :D
~ ¡¡ sorry for the wall of text but can empathize and wanna help !! ~