While I agree with your reasons that the dog would likely be better off staying with OP's parents, I can't understand why you phrased the bolded like you did. It implies that the OP isn't putting the dog first. Everything she's written leads me to believe that putting the dog first is exactly what's happening. Even the sentence "Or if I should wait until September, which makes me feel guilty about being away from her that long," suggests that the OP is focused on the dog, not herself. (Then it would have been something like,"Or if I should wait until September, because I miss her and want her here.")
OP, there's nothing wrong with choosing to keep your dog where she is being well cared for, even if it isn't with you. Ignore those trying to make you feel bad about your decision. It must be so hard to not have her with you, but you are showing great love for her by having her stay with your parents. I would probably have her stay there until September, so when she does arrive you know you will be able to give her more time and attention.