I have trouble with "letting go" too and that includes just about everything in my life from people to old clothes. I am a lousy spring cleaner because I hate throwing things away but I am getting better at it with practise
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I allow "reason" to guide me and not my emotions.
"Toxic" or "one sided" relationships are not much of a problem for me anymore because I know that ultimately somebody's feelings are gonna get hurt (usually me) and I can deal with a little "pain of separation" now than with a "broken heart later".
I have some tight rules for myself now where my relationships are concerned. For the first time in many many many many years I am finding a little peace within myself and I will let nothing or noone disrupt that peace.
Focus on your work and on your recovery. You will find new friends. One day you will understand the power in the word "goodbye".