I wouldn't say changing is bad while in a relationship, it honestly depends on how it impacts the relationship and what that change is. if it's a toxic change, then not so good and you should figure out what has changed or happened to give yourself a toxic trait. The best thing to do if you notice a change, good or bad, is to talk with you partner about it :3
Personally speaking I've noticed I change for the better, but I've had my struggles in previous relationships.

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