Explaining to my boyfriend that my orgasm isn’t about him

Let me explain, because the title sounds misleading.

I love my boyfriend. I love sex. I love having sex with my boyfriend. But I don’t always orgasm from the sex we have. Sometimes I’m stressed or tired or not in the mood and sometimes the sex may have just not been that great. But the majority of the time, I do enjoy it, I just don’t orgasm.

My boyfriend (who orgasms every time) is very attentive to this, and does everything he can to change it. He’ll give me head for as long as I want and touch everywhere I say but often, it’s still not enough. He’s gets kinda down about this sometimes and says that he feels like he’s not pleasuring me enough or that he’s not big enough or something, and I try to reassure him that that’s not the case. Occasionally he understands, but it’s still obvious that this is really affecting his sexual confidence. How do I help to boost that and explain to him that it’s not his fault?