My first instinct is that a long letter coming clean isn't likely to go well.
That said, I would highly recommend you draft it up, with what you want to say... .and post it here for review.
The process of writing it will probably help you work though things you need to work through, even if you never deliver it... .and we can review it to make sure you don't just throw a burning pile of invalidation in her lap.
The tricky thing about old, existing lies is that they tend to need a LOT of "maintenance" - the longer the old lies sit out there, the less energy I'll have to deal with current real life situations.
I think you can sort those out when they happen. You will hit a specific time and place when you need to make a "maintenance payment" on one of them, and you will decide that it is time to come clean on that one instead of maintaining it.
Chances are the cost of coming clean will be a fight that you will have to end by enforcing a boundary. And at the time, it will seem a better price to pay than trying to maintain the prior deceit that was supposed to keep the peace.
So deal with them as they seem right when they come up... .I normally advise against looking for that fight; trust that it will find you soon enough.