That's not nice that he broke the plans with you to go to the beach and then turned around and went with his friend, instead. I'd see it differently if you were glad to get rid of him for an afternoon, but it sounds like you wanted to resolve your fight and really did want to go to the beach.

You clearly wrote that you are together 24/7. I'll confess that that can be too much for many couples. It's sometimes good to have some time without each other. That can make seeing each other again a real treat. Perhaps he yearns for such time on his own or with a friend. Please don't take that as a rejection because it's a healthy thing. If my husband goes out with a buddy, I try to savor the time or make a date with a friend/relative. Sometimes I go to a restaurant I'm comfortable going to alone that my husband rarely wants to go to. Or a movie he'd never want to see.

Do you have a therapist and/or a psychiatrist? Have you talked about this issue with them? They may be able to help you find ways to give each other space in comfortable ways.