I wanted to wish you all the best for tomorrow Jen! Despite the anxiety and nerves (which are totally normal) , I get the impression you're feeling more positive about this which is so nice to see.
You're going to be just fine. One thing I can guarentee you, having gone through all the same anxiety you're experiencing is that the injection will all be over with very quickly.
The 15 minuet wait is a bit scary, yes. But allergic reactions are so rare that you've got a greater than 99.9% chance of being totally fine. The main battle will be controlling anxiety and the worry that something bad will happen. But it won't, you'll get through it, and you'll be really proud of yourself for pushing past the anxiety and getting it done
Hope it goes as well as it possibly can, I'm sure it will!