I have absolutely no idea, yet. I have a few of those loose ends myself. It is painful. Painful to a point of being the point around which my entire life obssess some days.
However, I have noticed that with time the pain and rawness diminishes. The frequence with which they pop into my mind or ambushes me unsuspected decreases.
Every now and then I will wake up from a dream in which I encountered them in a positive manner. F.i I'd dream of a chance meeting somewhere where a conversation took place in which I could tell them in a good way how I felt, and they responded in kind. At waking up, my conscious thought of them would be a brief glimpse of acceptance that they/I had moved on and I, just for a second could let them go.
Over the span of my adult life 30+ years I have learned that at the end of a relationship where there are unresolved stuff - it ended badly, one party just walked away without a word, life took a weird turn and contact was just lost - we actually need to grieve as if the other person has died.
Something I have done to facilitate a concrete, hands-on resolution is letter-writing. Letters to the person whose loss I am grieving that I never sent (I then threw the letters in the trash or burned them). Simply telling them what I feel, what is on my mind in regards to the lost relationship. It did help to put stuff in perspective, sort out the raw emotions. I am not done yet. I still have painful ghosts lingering, but I am moving towards peace of mind bit by bit with each of them.
There is hope.
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Looking or asking for a cure for Autism is like looking or asking for a cure for being Black.
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there, their, they're - learn the difference goddammit!