I can't vote really on that one. I think (personally) vet would be a great way to go. Because you'll get lots of education you can always go number of different roads: for example, I know a vet who became FDA employee (she was hired because she had a degree). Also helping out creature in need is just very rewarding IMO.
The downsides are 1) it's very expensive (meaning you'll owe lots of money) and 2) whether you'll be able to study. Please don't get me wrong on 2nd one, not-so-good grades in HS doesn't mean much. Vet school is lots LOTS of work and study in different fields (including bio, chemistry, and such) and you have to judge yourself whether you can really spend so many years studying hard (and very likely will have to combine study and work to make at least some money to live).
Being the farrier is certainly very profitable in my area.
The downside of that is 1) you need to find new clients and start trimming for cheaper, 2) even with the school you better partner with the experienced person for while trying to learn as much as possible from him/her, 3) dealing with the hoofs (especially the back ones) is very dangerous. Not every horse is nice and sweet, and most of the time you can't pick the customers (or you'll be out of them too soon). Also 4) farrier work is really tough on your own back as well as you need to be strong enough to deal with the weight.
As for massage therapy, well it doesn't sound like big thing around here (I'm not sure about where you live). And personally I think chiro/therapist MUST have vet education (at least the basic one) and I wouldn't invite one without to my own horses.