my relationship

My boyfriend ‘25M’ and I ‘18F’ have been together for 1 year ,we live together for almost 2 months now ,and we have a very active sex life. However, for about a month and a half now, I’ve noticed that during sex he doesn’t finish anymore and we don’t sleep together that often anymore .
I talked to him about it, and he told me that it was because of stress and some problems he has been dealing with. A few days ago, we tried again, and once again he didn’t ejaculate. I became frustrated and asked him if the problem was me, or perhaps the way I look. He told me that it wasn’t, but then said that I could gain a few more kilograms.
I then asked him if he had experienced this kind of problem in previous relationships, and he told me that yes, he had, in all of his relationships except for the one with his ex-girlfriend. Hearing that made me feel extremely bad, and I didn’t say anything else to him.
A few days later, we were having a meal together with his family, and a 10-year-old girl who is the daughter of someone he knows called me by his ex-girlfriend’s name. I didn’t say anything, and nobody seemed to hear it. After we got home, I confronted him about it, but he told me that I must have imagined it and that I was making things up for no reason.
TL;Dr Could you please help me? I honestly don’t know what to think or how I should feel about all of this.