I need someone's advice ughhh

I'm so sexually frustrated idk why I can't cum when I'm alone I can but when I'm with my bf I just can't no matter if it's penetration or playing with clit. I just can't cum and I feel like it's one of those things where I'm thinking too much about it and I'm like constantly thinking about cumming that it won't happen. How can I fix this? We have tried different positions but it doesn't change anything. Give me some advice plssss