Why am I getting blisters from my soccer cleats?

2 yrs. ago I bought some Adidas soccer cleats. When I started getting blisters from the shoes, I thought it was just because they were new. But now 2 yrs. later they are still giving me the same problem. I usually wear size 7- 71/2 in regular shoes. But because soccer socks are a bit thicker than normal socks I bought size 8 shoes and they fit perfect (my feet were not squished). I decided I couldn't take the pain anymore and bought a new pair of Puma cleats in size 8 and now I am having the same problem. Blisters on my heels as well as lots of pain and tenderness on the sides of my big toe and the ball of my foot. I am frustrated and don't know what is going on. Am I buying the wrong shoe? The wrong size? or is there something wrong with my feet, besides the fact they are so sensitive. Please help! I am about to return these cleats.