I can’t cum😩

My boyfriend and I have been together for 3 years we have never before had a issue in the bedroom....until recently the last 2 months when we are having sex I won’t be into it as much and I don’t know what to do!? I love my boyfriend and can’t imagine myself with anyone else we have tried to spice things up and trying new things to see if that will work but it’s not working, and I can see it’s starting to effect him as he’s thinking he’s not good enough which i do reassure him it’s nothing to do with him as he’s completely fine( he is understanding with it and says he’s fine but I can tell it’s making him feel down about himself in the bedroom) i will be really horny at the start during the foreplay and during sex but then sometime during having sex I kinda just loose the feeling and don’t know how to get it back 😩🤷‍♀️ my question is how do I stop myself from stop being horny halfway through sex??

I’m not thinking about anything else during sex.

I’m not stressed everything is going fine in life at the moment,apart from this😩