Give yourself a break sweetheart it's only been 3 months, I would feel exactly the same and I can't stand my ex!
This person was a massive part of your life for a long time and I bet you had stuff planned for the future and about how things were going to be. Even though you're both moving on that stuff is still going to be part of you for a while.
You are perfectly sane, frankly if after six years you weren't a bit upset by this I'd worry more. Perhaps give yourself the night off, curl with a glass of wine, have a good cry and then put on some really cheerful songs... and remember all the good things you have in your life still.
Chin up hun, you'll be ok xx