Well, on the bright side, you’ll never have to wonder what “painting yourself into a corner” means. Unfortunately, you have to get out of that corner now, and there’s no way to do it without making a mess. The best you can do is damage control.
When you do fess up — and I agree that you should — he will be upset, but all you can do is be honest and apologize. He will either forgive you or he won’t. I’m not sure why you started faking in the first place, but my guess is that it just made things easier. But lying is only easier in the short term, and becomes a huge pain in the ass the longer it goes on, as you are discovering.
Because you’ve been dishonest, he has no idea how to please you, and the two of you will have to start over from scratch in bed. His ego will be bruised, yes, but his trust in you will also be shaken. If and when you do start having genuine orgasms with him, he might still think you’re faking it, and you’ll have to convince him otherwise. Your confession and apology to him are only the beginning of the work to be done.
But.. I’m not here to scold (much). What’s done is done. Chalk it up as a life lesson and move on. It might help to remember that your inability to orgasm during intercourse without clitoral stimulation is fairly common, and doesn’t make you abnormal or him a bad lover. His only offense is believing that the female body isn’t complicated. He’ll be bummed when he realizes that he doesn’t have a magic penis after all, but he’ll get over it — all men do.
Starting over isn’t necessarily a bad thing, either. Being honest with him will be a relief. You can teach him exactly what it takes to please you sexually, and he’ll be a better lover for your instruction. It’s a win-win, and when you achieve climax, you can both be proud of the accomplishment.
Here’s hoping you can both get over this obstacle and be better communicators — and lovers — because of it.