A few things to consider from someone who has dealt with hundreds of men using such injections:
(1) Start with a dose that your doctor suggested based on an office injection and increase by increments of .02-.04mL until you reach the desired effect (not more than 1 injections per 2-3 days).
(2) Priapism can result in permanent scarring and should be dealt with one way or another. Docs recommend Sudafed as a first line treatment, but it doesn't have much effect. You can get a RX for a Phenylephrine Kit (1mg/cc 5ML Sig: Inject 0.25ml every 15 minutes until erection comes down), which might save you a trip to the hospital if it comes to that.
(3) Some men experience nodules on the shaft at injection sites. These will eventually go away but it may take on the order of years. Best to vary injection site, both right and left side of penis and up and down the shaft.
(4) There are many possible formulations of injections and you should feel encouraged to try different formulations if you feel you are having to inject too much or it is ineffective. The injection mix with the least pain is usually papaverine+phentolamine, and that can be done in various ratios, too.
(5) You might try adding a vasoconstriction device (basically a ring) to prevent venous flow if erection duration is insufficient.
(6) Your doctor is probably too busy to optimize the injection process for you, so contact the compounding pharmacy and ask their opinion when you feel a change is needed.