Sex really hurts in the beginning

Ive been with my bf for 3 months and the first two months sex was fine. It didnt hurt for the first few mins like it does now. I clench up and automatically try to move away when we start and it takes almost a minute to feel any pleasure. We use lube now but its not helping. We hit a rocky point in our relationship about when this started and it was emotionally devastating to me. Im better now but still emotionally coping (he had a baby two weeks ago out of nowhere and he has sole custody now and a 1 year old and my 4 yr old twins i have custody of. I felt a lot of responsibility fall on my shoulders with diapers and bottles and transportation and income mainly comes from me) is this the reason im tightening up when we have sex? Is that possible? How do i stop it from happening. Its causing strain in our relationship cuz i dont want to have sex anymore cuz the pain is extreme. Help?!?!