Ladies please help

Okay so lately my fiance hasn't really been initiating sex and when we do he can't seem to finish. We change positions, do all the things and then he will either master bate after to finish or I catch him masterurbatung in the shower in the morning. I have tried talking to him about it but he doesn't really say much. We have been together for 5 years and did not have this problem before. Is it me? I am just so insecure about it now. It is one thing if he just has a low sex drive right now, but then why masturbate without me almost every morning. And how in the world can you be having sex that you tell me in the moment "oh baby this is so amazing" but not be able to finish? The whole thing is just stressing me out.