It does not sound like they are doing so well in their own lives, dealing with your eventual growing up and out and moving along themselves. I would do what you have to do, be as kind and truthful as you can ("Oh, I am sorry I cannot make it, I already have another engagement for that day!") and live your life as you would like. Maybe you will make friends you can bring home with you to your parents? I know that worked for me somewhat, I brought my dorm girlfriends home and my mother taught us to play bridge, which translated to fun in the dorm together too. Showing them that you are doing well, have friends, etc. might tell them it's okay to let go, it is no longer just them and you/you and them.