Not reaching orgasm every time

I don't cum/orgasm every time my husband and I have sex, I just can't get there sometimes and it upsets him because he wants to pleasure​ me and he thinks it's him, like I don't enjoy him anymore and that's really not the case because I do, even if I know I won't be able to cum I'm still enjoying him and the moment. I've told him over and over again that some times I just can't get there and that it's not his fault but he's still hurt over it.

How can I explain to him in another way that it just doesn't happen every time? To get him to understand that it's not him? I'm sure that I'm not only female that doesn't cum every single time and I've also explained that to him as well.

Please no rude comments just looking for some advice.