Hi OP, you absolutely have every right to pass if you're not in the mood. Since you've got a good partner right now, I definitely recommend you talk with him about it. Sit down with him at time where you're both fully clothed and in a good mood. Then, tell him what you've told us, something like, "Babe, because of my past experiences, I have a hard time saying no to sex, even if I'm not in the mood. You're awesome and I want to figure this out with you. So, if I'm not in the mood, how do you want me to let you know?"

One approach that I personally like is the rain check. Me & the hubs often say something like, "I'm worn out tonight babe, how about a rain check?" I personally like it because it's not just a "no", it's a bang-date rescheduling. It's a softer rejection, or something.

Additionally/alternately, you can always offer alternatives (if you'd rather do those instead - don't offer because you feel like you HAVE to do something). So, "Babe, I'm not up for sex tonight, but what about a handjob/blowjob/mutual masturbation/we watch some porn together while you masturbate/whatever else might sound good to you."

But really, anything you & your fella agree on is A-OK. So long as your meaning is clear, you're golden. Hell, you could have code words if you needed to. If your "no thanks" ends up being "Applesauce", who cares? The only thing that matters is that both of you know exactly what it means.

As for how he should react - a mature adult should be able to handle "not tonight" with maturity. When I tell my husband "not tonight", his response is, "Okay, wanna watch TV?" or "Can we plan on tomorrow, then?" It's not a big deal. If the overall quality of your sex life makes both of you happy, then the occasional pass is a non-issue. And, if the overall quality of your sex life isn't making you both happy, then that warrants a calm, adult conversation.

If he gets a bit bummed out, that's understandable, but getting angry, sulking, yelling, threatening, whining... those aren't respectful responses.