I don't think you can cancel this transaction. The easiest way of dealing is just to wait till it goes through. I watched some videos on how to add some fee later, but the ways described there seem not to work anymore (btc was upgraded some time ago and now it's impossible). The only thing you might try doing apart from waiting is trying to broadcast the transaction, let people (miners) now that you need help with it. Watch this video: https://www.youtube.com/watch?v=dZSEPLpqNA8. I advice not to deal with other websites but the first part of this video is actually about blockchain and you might try that method.