Well, the beginning of your post really says it all for me, I'm not interested in dating. You don't seem to be pining away for a relationship and that is fine. Really. Knowing who we are is a good thing. You may be someone that finds someone down the road that you can have a companionable relationship with but the full blown, ball and chain thing might not be in your future. Which, again, is okay. Maybe you feel like you are 'supposed' to have that kind of relationship but you are just supposed to try to be happy. And if being your own woman, untethered makes you happy, that's just a fine way to live. I was single for many years, well into my 30's. I enjoyed career, friends, hobbies. I decided that I did want children and the family life and then it happened. But, it could have gone the other way. As long as YOU are happy, it doesn't matter if you have the traditional relationship in your life or not.
Are you happy? If not, or you feel that your pattern is hurting you, then see a counselor to discuss it. That's a great way to explore why we do what we do. good luck
Maybe it's just your personality type. Maybe you're beating your partner to the punch and defeating the fear of being rejected by getting bored sooner and wanting out. Maybe this kind of timeline is all you expect of relationships, or is all the effort you want to give to them. Maybe the people you are choosing to partner up with are good at being romantic but poor at being deeply caring in the long term. Who knows. For you, only the frothy, early stage of relationships is enough to keep you involved. It might stem from what you decided about the world when you were little. But if it is exactly what you want and it satisfies you, you might simply stop worrying about it.
There is nothing wrong with creating relationships that make emotional sense to you, as long as you are honest with your partner that your interest will probably be short term. The main question is whether you are happy the way things are. If you are not happy with it, talk to a therapist and try to figure out why you check out emotionally early in relationships. You could explore whether what you identify as "independence" is you not wanting to connect in a committed way, and why that is.
It's all fertile ground for inquiry if you are curious. But it is not necessary to inquire, if you are happy with how your relationships go. Just expect to have flings and continue the way you do. (Just be honest with the prospective partner.) Lots of people have serial relationships, you aren't the only one.
I agree with both comments you've already received fully. It's fine to know what you want to be happy, it's important early on to be honest about who you are to those you do choose to date even for the short term, and there is a therapist always to help you navigate through your innermost feelings, conscious or unconscious. I'm curious, do you enjoy long term friendships (as those would be considered companionship).? There is also an option for you to seek out likewise thinking individuals to develop long term companionship with. I'm quite sure there are others who feel exactly the same way as you, and seeking them out to include in your life might give you that "joy" that you believe might be missing in your life.