Hm. Well, I called today, and the pharmacist said using it internally can cause low blood pressure and headaches, but it's okay if the doctor said I could use it internally.

He also told me it WOULDN'T increase my heart rate - if anything, it should lower it, so I have no idea what's going on with that... I googled "nitroglycerin increased heartrate" and found a bunch of websites that said it was a side effect, among many others. There are certainly more side effects than just the headaches, which is all my label warns about.
I guess I'll keep using it externally for a few days and then try using it internally.
I'm kinda feeling resigned, though, and think I'll have to get the surgery anyway. Based on what I've read on the forums, it seems like once the fissure has lasted more than six/seven months, it might heal for a time with the cream, but continually retears until you have the surgery.