First don't be so hard on yourself.. ALL snake keepers make mistakes! You took your snake to the vet.. that's something that a lot of half hearted snake keepers won't do.

I have given a lot of shots to snakes. Generally, they don't mind it much. I've given shots to my 5.5' hogg island boa, and my 7' 21 lb dumerils boa by myself. Generally you give shots in the first third of the body on either side of the back bone. I just support the snake from below where I am going to inject, and be sure to go between the scales, as you can break a needle on a scale! Just do it quickly, and you should have no trouble. If she's really feisty, restrain her behind the neck so she can't bite, and inject as usual.