I think there's a couple of different things to think about, that might help:
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Loving, forgiving, and accepting your past self for their actions - whether you did something bad, cringeworthy, or something you've regretted in a relationship, you're a person doing the best with the tools you had available.
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Recognizing that the positive emotions you remember are the way that you-at-the-time felt, not the way you-in-the-present feels.
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Acknowledging that you're thinking these thoughts, but choosing not to pursue them. There's nothing unhealthy about a wistful memory popping into your head, but it's maybe unhealthy to pull up their fb and check on them, or otherwise extend it. There's nothing wrong with thinking "Yup, I'm thinking that, but I'm choosing not to pursue it".
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Understanding that how you feel now is temporary, not permanent.
It takes time to heal from things, but that's ok too! Best of luck!