Losing erection when out of breath

I’m sure there’s a few things that go into it (stress/anxiety, poor sleep, occasionally alcohol), but when I lose my erection it happens at the same time every time.

I can always get hard, and we’ll usually start off in missionary, but after a few minutes when I start to get out of breath and need to change position , this is when I’ll go soft.

It now feels like I need to cum in the first position of sex or I’ll lose it.

Has anyone else experienced the same/have any advice?

I’m fairly fit, but plan to up the Cardio to at least make that point further away so I’m more likely to finish before my lungs do