Well, there are a lot of injections that can be given at home. I don't know if DMPA is one of them (I think it is), but I took hcg injections at home, and had to mix them myself.
You might consider the pre-mixed syringes instead. They have a shorter shelf-life than the solution you mix, but you won't have the problem with them separating.
I would call your GYN and ask. Dont let him dose you again out of paranoia... make sure its from the medical standpoint that it would be safe.
If your fiance can't answer this one for you, I'm not sure we can help either. Have you called the pharmacist?