Well, the advantage to sticking it out is that you get to stay with the person you've loved all this time. Only you can judge whether that is worth the pain and struggle of getting there (and I absolutely mean this without judgment - it was for me, but it will not be for everyone). There are people who find that their sexuality can be flexible enough to accommodate making *one* exception for a beloved partner who changes, but there are plenty of people who can't, too. And HRT may in fact make him* unable to function in the bedroom as you would like, although it may not (the catch there may end up being whether he's willing to do so).
It is NOT that easy to adjust, and he needs to stop pushing you to be fine overnight. You can even tell him that doing so is counterproductive, because the more you feel like you "should" be OK instantly, the harder it will be for you to deal with the actual process. You are not responsible for his happiness, either, and he needs to stop guilt-tripping you about being *un*happy - that's a perfectly reasonable and normal response for you right now!
For now, and I remember myself what a fog of fear and depression and misery those first few months are, just be gentle with yourself and try not to worry too much about the distant future (if you can, I know it's easier said than done). Focus on coping with the immediate stuff, and so long as the answer to "do I want to leave right now" is no, keep trying to adapt. If that answer ever becomes yes, of course, you are well within your rights to head for the door.
What I did a lot of the time was to give myself "permission" to cry and grieve for the loss of characteristics and other things that I had loved, and to remind myself that eventually this horrific period of constant change would have to be over. Yes, it'll be the same person; no, that doesn't mean you're shallow or weak or silly for caring intensely that external details about that person are changing. It's human nature. You're going to be angry, and hurt, and frantic, and a million other emotions, and the only real way out of all of that is through (or divorce, if you choose).
In general, I second the recommendation that you keep communicating, keep trying to be loving to each other, and stay committed to trying to make this work (for as long as you can). After 20 years and 5 kids, you obviously have a lot of love to build on and plenty of tools for keeping a relationship working to fall back on.
*hugs* My wife transitioned in 2009, 11 months from figuring out what she needed to do to being full-time and done, and while we're happier together than ever now... it was a hell of a year. I cried off and on for days when she had her ears pierced. Last summer, she had bottom surgery and it didn't phase me at all. It took a long time and a lot of work from both of us to get to that point, but it DID get easier. (For the record, I ended up becoming good friends with another wife who was there to support her spouse through surgery at the same time I was, who was in her late 50s and totally straight and realized that - with the same kind of dedicated effort and time - they could preserve their marriage and their sex life. Not everyone can, again, but it's happened.)
Good luck, and remember to take care of yourself too.
*note to everyone else reading, and I think I'm making this a standard disclaimer : yes, pronouns are a minefield. (I remember a period when seeing "she" used before either of us were ready hurt me acutely, so I'm gonna err on the side of being sympathetic to the person writing this, if I can't otherwise determine.)