U seem to be suffering from performance Anxiety. Due to overthinking about ur ability to satisfy ur partner, ur performance goes down, because sex is a mind game, needs to be done under relaxed state, not under any pressure or tension.
Next Steps
Whenever u go for sex, tell ur partner that u r not going to put ur penis inside, u r just going to enjoy foreplay. U can do foreplay for 15- 20 mins, whenever u feel enough confident, u can put it inside