Tired boyfriend

My boyfriend and I have been dating almost 5 years and we have the most amazing relationship. Everything is great for the most part. The only thing I can complain about is our sex life. We make out and foreplay a lot. But he's often "too tired" or "too stressed" for sex. And whenever we do have sex, his mind just isn't there and it takes him over an hour to cum. I then get so upset thinking it's something with me, and he gets upset that I get upset. It's a mess and we've only been having sex like once a month and each time he either doesn't cum or takes an hour. Any advice on how I can fix this or what this might mean??? He swears that he still loves me and it's not anything I'm doing and that he's just stressed and tired lately... Idk what to do!! I miss sex!!!