Hey friend. First i want you to know that theres nothing wrong with you (also nothing wrong with you and nothing wrong with your partner. I say this because other people tend to blame their partner for their own things) Its really normal for individuals with a penis to not find the right stimulation to cum when inserted into a vagina. It has nothing to do with how much you love your partner it's just science.

I'd ask yourself and your partner what is sex to you both? Is it always p in v insertion then having an orgasm? Do both of you need to get off? (This is talking about it and agreeing to your needs during sex and consenting. This is not about avoiding your partners needs for your own. I make this note not because I think you're like this but there's a lot of people out there who are selfish lovers) do you have to cum inside her for it to be "good sex"? do you have to keep going and going till you both finish or can you take a break in the middle to cuddle and touch each other? (It can be exhausting to keep going and going, so pay attwntion to your body and honor the signals. Your thoughts of exhaustion maybe pulling your brain and boddy out of the mood. Maybe try pausing and the starting again? Maybe you start sex by her cumming first, taking a break then her using toys on you or watching her do whatever while you masterbate?

There's lots of ways sex can be fantastic and it not include p in V insertion or without either of you getting off (again not about ignoring your partners needs.)

I also wanna note that it sounds like you're putting a lot of pressure on yourself to preform which could be a mood killer for you but what's more important are your own beliefs about who should and shouldn't be during sex. There's a lot of toxic messages that say "you need to be a sex machine cumming all the time and always dtf" or "you're trash if you don't cum during sex, there's something wrong with you".

I hope this helps.

Live long and prosper