This is porn/masturbation related first and foremost imo and in 2 different ways. I've looked back at my own experience and i think i have a really good picture of exactly what is going on. Nofap communities have a good idea of whats going on but i think i've pinpointed more precisely the mechanism of this and as such also found the fastest way to fix this with what i am going to share, so you can get back your normal function as fast as possible.

First to share that i've also experienced it. I've definitely been with a girl i've been attracted to and completely unable to get hard when we are both naked. And a few weeks prior to that i could get painfully hard just thinking about her. And you know what was the difference between those instances?

I was getting painfully hard just thinking about her about a month into no porn/masturbation.

And what was the difference when i was with her and unable to get hard? I masturbated to porn a few times the week prior.

Now i am not saying masturbating a few times the prior week to having sex will make every man unable to get hard when with a woman.

But a man that has conditioned himself most strongly sexualy TO being alone and masturbating with his hand and with porn/nsfw content? Well that man has conditioned himself to something very different to sex. So when sex comes around, his body is just not responding. The body wants what it wants and that is the thing you've given it for YEARS prior. So even if you get a nice nofap streak going, the moment you do a few fap/porn sessions you wake up your strongest sexual neural pathway in your brain, which is the lone masturbation/porn one.

Porn/nsfw content affects what you get aroused by, i.e. the screen when you are alone and looking at images/videos of naked women engaged in certain behaviour. Likely also while sitting/laying and touching yourself.

Just quitting porn forever long term (and you should) should fix your ability to initialy get your erection when with a girl but then your ability to HOLD the erection and able to fuck IS A SEPARATE THING. And this is what i think most nofappers are missing.

To not lose your erection comes from having your brain wired to enjoy that type of physical stimulation/situation but currently your brain is wired to pump blood into your penis when its getting stimulated by your own hand motions when you are alone, likely when you are sitting or lying and looking at nsfw content. It is the same thing as death grip but you can get that even if you are not "gripping your dick very hard".

My advice is: Completely quit porn and any type of masturbation/penis touching for 2 weeks (apart from cleaning ofc). 2 weeks should be enough. Forget the 90 days bullshit. Its not realistic anyways.

This will probably let you get an erection when alone with a woman, probably even before you guys start undressing yourself but definitely when you get naked. But then you are likely to still experience losing your erection (or delayed ejaculation) when you get to penis in vagina sex because your brain is wired to getting stroked and more specifically BY YOUR OWN HAND.

But just going and going with nofap and waiting for all of this to reverse by itself takes too long (thats why nofap communities preach the 90 days but that is too long).

Now, of course, if you can find a willing partner who will be understanding and help you with your re-wiring to fucking, that can work, but its not terribly reliable.

What i did, and if you google reversing death grip, other people do/did, is buy a fleshlight (yes i am serious) and start fucking that with lube every day for a few week(s). Even 1 might be enought.
(read some reviews and get the one that is most similar to a vagina feel, cuz some of them are unrealisticaly tight, theres different "models" with different "insides".)

The key thing here is DO NOT hold the fleshlight with your hand AT ALL when you are doing this. Either pin it down or get it stuck somewhere and fuck it like you'd fuck a pussy and try to finish that way and again NEVER HOLD the fleshlight with your HAND as that will still reinforce your "own hand motion stimulationg own dick" naural pathway.

You will notice that if you just hold the fleshlight you immediately feel more pleasure, and especially if its your hand moving the fleshlight instead of you fucking the fleshlight its WAY easier to cum.

Why? Because of everything i just explained, thats what you've conditioned yourself to. To your own hand moving up and down on your dick and thats what gets you off AND NOTHING ELSE can get you off anywhere as easily.

So you must stop doing that completely and instead start doing WHAT YOU SHOULD BE DOING, which is the fucking.

After just a like a week or 2 (depending) of fucking the fleshlight (and don't forget lube) you will be able to maintain your erection fucking the thing and you can move on to sex.

I suggest hiring an escort once after that, as its (at least for me), less performance anxiety inducing so you are eliminating that possible cause and making sure that at least physicaly/brain wiring wise you are good to go.

So next time you are with a girl, you will be pretty confident everything will be fine and even if something happens AT THAT POINT, its probably just some mental thing which you can also simply get over with more attempts/experience with real willing partners.

One last thing is. You must completely stay away from cumming by your own hand even if with a partner. What you are likely to experience is that even if you CAN hold your erection, you are unable to finish by just penetration initially (Delayed Ejaculation basically). You MUST resist the temptation of your or your partner's desire to find a way to finish in any way possible which will probably just involve your hand...again that will reinforce the wrong pathway and you will likely get limp dick again on your next sex session. This all might mean that you will basically also be practicing semen retention since you simply might be unable to finish from fucking. Don't try to force it. It will come eventualy. Semen Retention is even yet another thing that will speed up your recovery and restart your libido anyways.

I am not saying this will have to be this way for the rest of your life BUT you must stop getting your dick jerked and build up that brain wiring for fucking for a decent while.

As far as getting sponatenous erections/morning wood, that will improve over time as well the longer and longer you stay away from porn and hand masturbation but don't get too fixated on that. Just get to the point where you can hold your erection fucking a girl and over time you'll build your confidence in your tool working and so your frequency of morning wood/spontaneous erections will be irrelevant.

Hope that helps.

EDIT: One last thing, also probably should avoid cowgirl positions for now and instead be on top and fuck the girl, its def easier to lose your erection when shes on top imo, especialy in the still shaky erections period, eventualy you will get wired to real sex enough to where any position will work for you but make sure you can get it working in the more basic positions first ;).