A.Z.
You need to have your doctor run a full CBC blood panel and tell you exactly why you have low blood platelets. It could be something simple like anemia (very common with pregnancy) or an early sign of preclampsia among a dozen other things. Most likely it's not a serious issue like your thyroid or he would have ordered more tests and discussed this more thoroughly. But you need to be your own advocate! You can't deal with it until you know what is wrong. There may very well be a way to return your levels, your doctor just isn't used to trying. That is why I use midwives personally, they are proactive and focus on preventative care rather than treating issues after the fact. Just a different style that works better with my personal philosophy.
ANyway, you are also suffering from the all too common pregnancy brain! Keep a pen and paper in your purse and write down your questions as you think of them and then go over them with your doctor at your appointment. DO NOT let them make you feel rushed or like you are bothering them. You are paying them for their services, get the answers you need to have a safe and healthy pregnancy!
If you really want a chance to have a successful VBAC, I highly recommend Andaluz Waterbirth Center. You can go in for a free consultation, they will answer all of your questions and you'll never feel rushed or unwanted. You can take as long as you need for your appoinment, mine averaged about an hour each time! It's worth looking into.