Losing erection, or short erection, or quick ejaculation

Hello. I am new to this forum. I hope I can find guidance and help what to do. Excuse me if I use improper language - I hope I will get notificaiton for this, so that I can re-write my post in a more suitable way.

I am a 31-year old man. I identify as gay. I have not been with many men in my life, so I don't have much experience, especially in sex. I've had only one long-term relationship, but it ended soon, because of the issues I will write, which I need help for.
I have troubles with erection. When I am alone (masturabting), I don't seem to have an issue. I can get a proper erection every now and then (but not every day, I am not that sexual).
However, when I am with a partner, my performance is low. I can't get a proper erection, or, even if I do, it is short (even during the pre-sex game, and not the actual sex itself). Even if I happen to keep the erection, it goes away prior the penetration part itself, or when I put on a condom. When I masturbate, sometimes I put intentionally a condom and I don't have an issue. But when I am with a partner, things don't happen. And finally, even if I manage to do the penetration, I ejaculate too fast.
All these ruin the pleasure of sex for me and my partner. I've tried to make sex several times in my life, and every time it is the same. Only with my ex-partner it could happen, but again very rarely. The lack of sex ruined the relationship, and I am afraid it would do so to every other I try to have in future.
Lately, I am trying to do exercises for strengthening the erection. Like kegel, eating proper food, which supports sperm-production, doing more exercises for endurance, edging, masturabting less (once a week), not watching porn, and such. Then I tried to make sex two or three times, but the results are the same. I don't know if it is because I don't masturbate the proper way (the head of my penis is very sensitive and I don't stroke it, because, if I do, it gets hurt; also, when I was a teenager, I would only rub my crotch and ejaculate, instead of stroking), or because I am in my head when I am with a partner, and it affects my performance.
Several years ago, I went to a doctor, but he prescribed to me some pills (I've tried two types of them). I would not like to take them again, because I lose feeling in my penis, when I use them, and after the sex I have terrible stomach and headache. I feel like going to faint and need to lay down.
I also tried to be the one receiving the penetration, but I didn't like it, so it's also not an option (I mention it, because often people tell me this as an alternative solution).

Could you please advice what to do? Or do you need more information?
I am starting to think that it's already a psychological issue, rather than a physical disfunction of the penis, because I am satisfied with my masturbations, but not with the actual sex itself. I would like to fix it, because it is an essential part of a flourishing relationship, and I have a goal to create such for me and my future partner.

Thank you in advance for your support.

NiceGuy88 wrote: [...] Even if I happen to keep the erection, it goes away prior the penetration part itself, or when I put on a condom. [...]

This is extremely common and I suffered from this problem for many years, it's called coital anxiety.

The good news: it is easily treatable.

You said you've already visited a doctor and that's great. Get a second opinion if necessary, make sure you consult a good urologist. You need to do a series of tests to determine the exact cause of your problem but even if this is just psychological, a urologist will be able to help you.

I also recommend you do A LOT of research on psychogenic ED, esp. sexual performance anxiety. When you get anxious or stressed out, adrenaline and other chemicals in your body constrict peripheral circulation and block erections ('fight or flight' response). This is a perfectly normal biological reaction, you just need to find ways to control your anxiety levels and the sympathetic nervous system.

Finally, make sure you are not taking any meds that may worsen ED (e.g. caffeine, anxiolytics, antidepressants).

Good luck!