I think you gave yourself the answer you are looking for. At 27 years old and with three years invested in this relationship, you are at the stage where generally people feel they need to make a decision about whether they are ready to commit. On the other hand, you have an idea that you are young and that you need time alone to come to know yourself. I think the reality that you can’t have it both ways is causing your feeling of “dread.” You have defined the situation in such a way that you either lose the woman you love or you lose your freedom to know yourself.
I encourage you to rethink that idea. Although one way of learning about yourself is to be on your own, another way people do it is within the context of a loving relationship. I hope you will reflect on how you came up with the idea that a relationship will confine you and prevent personal discovery.
I’m glad you are evaluating your attitude and that you are thinking seriously about your dilemma. If you can’t resolve it on your own, I hope you will consider seeing a therapist to help you take another look at your options. A person doesn’t need a mental illness to benefit from therapy. Sometimes a therapist can serve as an objective sounding board and can offer a new perspective to bump you out of your circular thinking.
I wish you well.