How do i make my hair grow longer, faster!!? And has anyone ever thinned out their hair? -Do you regret it?

This year so far, I've cut my hair 2 times! So right now my hair is a little bit past my shoulders and I have A LOT of layers. I am determined to make my hair grow longer in about 3-7 months. I want about 5-6 more inches. In this moment I'm using a shampoo that I bought at Navarro and its supposed to help your hair grow faster, but it was really cheap and I dont know if its going to work. I plan to thin out my hair as soon as it gets longer. Here's why... I have really thick hair (I have lots of hair) and I've experienced how hard it is wash and take care of it because the weight of the water adds up to the weight of my hair and its hard. So I thought if the shampoo really works by the time my hair is by the length that I want it at, I could just thin it out and it won't be that bad. So what can I use to make my hair grow longer and should I or should I not thin out my hair. Thank you :} <3