Needascream - it's difficult from this end to help, because (understandably) you don't want to give us too many details, so it's hard to know exactly how it happend, what was said, how it started, what he actually 'did' especially as the way you describe it changes (also understandably as you want us to understand without laying out the details so you keep trying). So either you can tell us the details (you are really double anonymous at the moment you know!!), or we can just keep trying to help but sometimes getting it wrong
TBH from what you have written I think that you do need to give your DH a break. He sounds sorry beyond words and what more do you really want from him? If you don't move past this, and quickly, it's going to cause a much bigger problem in your relationship than it deserves to. He made a mistake, it's one he's not going to make again, he wants to help, he wants to make it up to you - he wants to love you.... what else do you want him to do?? [I mean this with a not a ]
I understand that you were scared - but it was unintentional. Yes, he's a big bloke and could rape you if he wanted to - but in 15 years he hasn't - he loves you. He read the situation wrong once, in 15 years, he carried on for seconds longer until he realised you were serious/not into it - seconds.
Once, when I was with a guy I'd been with for a couple of months, I found myself in a situation where I panicked, he had me tied up to his bed (seemed like a bit of fun when we started!!) but I got scared, no-one knew I was there, I didn't know him very well etc - but at the same time knew I was being silly, I got him to stop, but it took a couple of minutes to make him realise I was serious and I was scared, I wasn't just playing along.... then when he did he was all fingers and thumbs and couldn't find the keys (yes ) and I was shaking by the time it was over, he was mortified I'd been so upset by it as it was just meant to be a bit of harmless fun - I think I made it a bit worse by telling him that it was just that I didn't know him that well, so didn't know him well enough to know he would stop.... we talked about it for a couple of minutes, both apologised, cuddled, talked about other stuff and went to sleep...
I am struggling to understand why this is so much 'bigger' for you than it was for me at the time. You say it wasn't as bad as you'd made it sound, but on the other hand you don't seem to want to forgive him? Is there more to this than you have said? (not that you have to answer that, just think about it!). That sounds like I think we should all be the same and I don't mean it like that, not at all, and I really feel for you if you are still upset/worried/scared/angry - I just can't, from what you have posted, quite understand why.
You have been with him for over 15 years - don't let a few seconds spoil your relationship x