Why can’t I make myself cum?

I struggle with cumming while having sex, but even when I’m doing it to myself, I hit a bit of a climax but can never finish it’s like almost too intense. And when my partner does it to me I have to stop him cos it gets there but then I’m like I can’t actually cum... and I feel like I never have... I remember once doing it to myself I did but only once and it’s been YEARS!!! Why is this. Is there something I could be doing mentally or physically to help me ?? I just want to enjoy sex and masturbation! Any advice welcome xx