You're grieving the death of your relationship, and that's every bit as traumatic as going thru the actual physical death of a person. A relationship is a real thing, with a lifespan, history, celebrations, and all the rest. Especially memories.
You might consider finding a grief group online, or perhaps a self-help book outlining the process of grieving. If it's a possibility, you might even see a therapist for several sessions to work through this grieving process. It's obvious that this relationship is monumental in your life, and until you have dealt with it, you may not be able to move on to other relationships.