I don't know if this will help, but all of this matters more to you than your partner, probably. How's the foreplay? Good foreplay can be an entire event, really. And if your partners have a vagina, and she's already reached orgasm, there is less pressure for you, right? If your partners have a penis, you can always use your fingers or a toy for prostate stimulation to make that happen, before or after, depending on which your partner prefers.
I understand it's not what you want, ultimately, but it might take some of the pressure off, easing what's happening between your ears.
Have you shared your concerns with your partner(s)? Ask them for other ways you might please them - fingers, mouth, toys, whatever.
And what is going on in your head? I'm assuming fear of not maintaining an erection, which then causes stress, which then makes your erection go away, so you're in an ugly cycle, but please correct me if I'm wrong. Is there a voice telling you that you aren't "manly"? If that's the case, women don't view manhood or masculinity through your penis. I don't know if your partners are women, so that may not be of any concern to you whatsoever, but I'd bet your partner, no matter their gender, would agree with me.
Are you allowing your partners to please you in ways other than intercourse? That may help, too, if you are open to those things.
Basically, my advice, as a 50something woman, would be to communicate and experiment. :) I'd also tell you to stop thinking, but I realize that's way easier said than done.