I need serious sex advice NSFW

So, I have a problem staying hard. I love my boyfriend very much and he turns me on, but for some reason when I try to put my cock inside him, right away it gets soft. Im able to penetrate him for a second but then it grows soft inside him. I'm not very fit, I have self esteem issues... can someone offer me tips?Also, I've been able to top him while we both lay sideways and finish inside him, but I can't say for sure whether I was full hard. Is there a logical explanation?