I understand your apprehension, however, it is inevitable that people will wrap up some of their identity in their partner. Inevitable, because identity is partially formed by experiences. Introducing a other person in your life, will make for experiences you would likely not had experienced, had you not been in this relationship. Ideally your partner will introduce you to new things, which you will either like from the start or learn to appreciate.
For instance, my partner is a formula one racing fan. This is not something I would have ever shown interest in, ever, had I not been in this relationship. But, since I am, it is inevitable I will become a person who has knowledge of formula one racing. I might even be a person who goes to races, or who roots for a particular team. I might even start to call myself a fan. Part of the labels I paste on myself; racing fan.
But is that me, or is that relationship-me? If, for some reason we are not together at the time of a race (idk, business travel) would I still turn on the tv to see it? And if so, am I doing that for me, or for the relationship? I probably wouldn’t think about this while still in the relationship — it would be just something I do, right?
But, if for some reason the relationship is no longer there, then I’d be forced to think about it. Was that me? Do I like racing? Or did I like the time I spend with my partner during the races? What would my relationship be to racing, if the person who introduced me to it is removed from the equation? I would have a crisis of identity.
And that’s just one thing. At the same time the same dynamic would be at work for all sorts of things. Music, food, spending habits, entertainment, maybe even the way you dress; if I had been hearing how well blue goes with my eyes, I am sure my closet would gradually fill up with blue. If I am suddenly no longer hearing that, I will have to ask myself; do I even like blue?
Relationships change you. That’s not good or bad, it can be good or bad but mostly it’s just change. Once that relationship is gone, there will be a time when you are confused about yourself. And you will have to figure it out. That can be very scary. Some people don’t deal well with uncertainty. That doesn’t mean they did something wrong.