Hashing a password, and summing it with every byte in a file. How secure?

I have a text document with all my usernames and passwords in it sitting on my desktop. I decided to spend a couple hours making that more secure. I wrote a program for the purpose. It takes a password and a file, SHA-512s the password and adds each byte of it to the file in 64 byte chunks. It subtracts to decrypt. I realise that this falls apart if two hashes sum to 256 for every byte and that this is also only brute-forceable if you know what to expect. Are there any other security flaws?