When you install FileVault you are given a recovery key which you can use to unencrypt your drive. You are also given the opportunity to store a copy of the key with Apple. If you did neither of these things and didn't keep a note of your password then I'm afraid you have a real problem. It's a bit like locking a safe and then throwing away the key.
Security software like this is useful to prevent theft. Your best bet is to contact Apple, or take your computer to an Apple Store, with proof of ownership. Be aware that they will almost certainly check the serial number against a list of reported stolen computers.
Filevalut is included in OS X, to the best of my knowledge it isn't available as a download; there is an "advanced file vault" (nothing to do with Apple), the question is what did you download and from where?