I thought about this a long time...only write a long letter if your husband is a reader. Letters, like e-mail are too riddled with other peoples' perceptions/emotions. If he's not a reader, you're going to have to suck it up and talk to him....but first decide what it is that you'e after.
Right now you're looking for permisson. You don't need it. You want him on your team. That's support. That's what you should ask for.
One of the most clear requests you can make is "I want your support. This is what I want to do. Can you to help me figure out how to make this happen?" You put together the requirements, learn whether insurance will cover it, and if so, how much. If he can't support surgery in Mexico -- and some people just can't get past that -- then make a list of other good doctors and where you'll get fills.
Is your husband generally supportive? If you wanted to take a class on something, is he generally cool with that?
If your DH said "I want to do the IronMan in Hawaii" or you'd probably plan together how to manage finances, schedules, space to make that happen. If he's a marathon runner, he spends $1000s in shoes and running clothes, entry fees, socks, what have you every year. They're his tools for success. Nobody runs marathons in sweat pants or flip flops.
He understands tools, he understands goals. He may have trouble understanding that you'll reach your goals with different methods and tools. What you're really asking for is for him to support you anyway, because the goal is the same, even if your methods are not his.
--and what Denise said below...we're here supporting each other.