Thick hair help

First, some salon options:
You can ask at a salon, they could thin it for you by cutting out some of the hair that's hidden underneath...I'll warn you though that a lot of people that have that done don't wind up liking it....and I don't think its a common thing to do either, so you'd have to talk to them and be sure you are confident they know how to do it right before getting in that chair or you could just come out with a haircut that will just set you back months.   Also if you ever wear it in a ponytail, or plan to soon, some of the shorter pieces may stick out. 

I know its hard to have thick hair when its in the in-between stage, but sometimes you just have to bear with it until it gets past it (then vow never to let anyone make it short again...that's what many of us do anyway if we don't like it short for whatever reason!).

Once your hair gets past your shoulders, you may find it too thick if its blunt cut (may make very thick ends and the top will lay flatter by then).  To avoid that you would need the last couple of inches (only from the shoulders down...not higher or it will stick out again) layered so they taper.

Some at-home-only options:
Another thing you can try without a trip to a salon is using oil.  You can use coconut, jojoba, or even just try it with olive oil you have in your kitchen to see if you like it. Use just one drop and spread it through your hair.  It will make it softer and heavier (and shinier).  Don't put it on your scalp, just your length, and if your hair looks greasy after you put it, that means you used too much, you will have to wash it and start over.

You can also try conditioner only washes.  No shampoo.  Conditioner will clean your hair, and it will weigh it down more than if you use shampoo and strip out the oils (thats what makes it fly-away).