Is this contract that was sent thru email legally binding and how do I know if this is harassment?

I was being paid to spend time with a man, with no sexual encounters involved, purely spending time together sharing a platonic relationship. I even have in writing via email him agreeing that any money, gifts, etc. were given to me for the time spent with him, and nothing else. I received gifts as well as money (there was no agreement that the money ever had to be paid back). During this time, I sent a few nude photos/videos to this man as a thank you of sorts for taking care of my bills. Our relationship ended, and he stated that he wanted me to pay back money or he would sell my nude photos/videos online. Because of this threat, I signed a piece of paper stating I would pay him X amount of dollars. I then sent him a photo of the contract via text message, and continued to make monthly payments to try to ensure he would not post my photos online. I now feel as though I was blackmailed, and that this contract is not legally binding so I stopped making payments and I am now once again being threatened by this man to be exposed, and that he is going to sell my nude photos/videos on the internet and send them to my friends and family. Is there anything I can do about this?