underhanded practices; loan sharks

I stopped using my credit card months ago after a family emergency. I have started paying more on my credit card every month without fail, doubling up two months to pay it down quicker, and once I did that they cut my available balance down so low that I look as if I've maxed out my credit card, which has brought my credit score down.  Everytime, I say I am going to double my payments to pay the card off and cancel them (partially because of their unexplained rate changes, they do the same thing with the balance.  They are underhandedly making it so that I cannot even use the card for emergencies or any reason.  I qualified for the credit increase or otherwise they would not have given it to me. I've never missed a payment. They are the only one of my credit card companies who have such shady practices.