Aww, man :(
A beloved pet dying is one of the worst feelings in the world. As somebody who has always owned rats, I know the feeling well. They only live two years and it never gets easier. There's something about losing a pet that just really breaks your heart. I think it's because they're so innocent and they bring out the innocence in us. It's like being punched right in the inner child when they go. Like a harsh reminder of how the world works.
A good pet absolutely changes us for the better and gives us many happy memories to look back on. He may not have been your best friend for your whole life, but you were his best friend for all of his. Take all the time you need to heal, never forget him, and remember that there are many other animals out there who need your love.