Hard Dick in the morning?

Is there a reason why my boyfriend starts getting soft if I tell him to go slower when we're having sex? and then he wants to stop and then 20 minutes later he wants round two which stresses me out because he gets soft and has a hard time putting it in sometimes even after he fingers me. Having sex with him is stressful period. And then in the morning he's super hard and wants to have more sex. .....why is he hard as a rock in the morning but when we want to at night he gets soft so easily?