Not sure if I can continue with this - please help

[CorsacReborn]

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I was as upfront and polite as I could be given the current circumstances and our past. He told me he thinks I am being unreasonable and that I don't trust him. I explained I trust him, but given the past with this particular person there is a justified amount of doubt there, which is a result of the betrayal and hurt. I explained it is only with this one particular person.

Just quoted this as your partner is actually correct here, you didn't trust him and that's fully understandable considering this person your partner knows is a "fuck buddy" from when you were first together.

Your doubt was justified but doubt rapidly forms a level of distrust of that person, as he's currently seen your point of view with the letter you wrote him and stopped communication with the other guy I would give him a chance to prove that he has taken your feelings into consideration.