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Honestly, I would just be transparent with the guy. If I was in this situation, I would say something along the lines of:"Hey, I feel like I need to be honest with you. I don't feel like I am as emotionally available as I could be with you and I feel like that isn't fair to you. You are a really good guy and I feel like you deserve someone who can give you their whole heart, and I still feel like there is a piece of my heart that I left with someone else and I feel like I need to work on healing that part of me before I go full-force into another relationship. I hope that you understand where I am coming from and I do truly wish only the best for you."
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I did an update
You have to let the guy your dating down easily and don't let it continue on. It's just getting worse day by day because he has no clue and thinks you're fully invested in him. Gently tell him it's not going to work. Don't tell him about any of this with the other guy because that will hurt him and make him feel used and worse. End it quickly but gently.
it’s just hard because this is my second relationship and my last one I ended very quickly and if I do it again it’s going to make me a bad reputation
Well, unfortunately you caused it, so you have to do the decent thing, bite the bullet and make it as easy as possible for him. He didn't know you had lingering feelings for some other guy. You'll live.
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