It sounds to me like through the past few years you've done really well at staying true to what you wanted (and didn't want) and what you were feeling at the time. I'm not hearing that you're somebody who has wanted to date or have a partner while liking the thought of it occasionally but are now getting curious about the possibility for real. Is that right?
I think there is a myth out there that 'healthy' people have 'healthy' relationships which apparently means finding a partner and forming a couple, that way all of us who don't feel that way are made to feel like there is something wrong with us, or to be told that we need fixing. So while I don't know what's true for you, I want to be careful not to agree straight away that fear of rejection is the only explanation. As you say, you're unsure about that explanation yourself, as though it's more of a mix.
I wonder if there might be a possibility you can experiment with dating without it feeling like you're making an irreversable 'jump'?
That would be one way to become closer to understanding what actually feels good, and if when you get past some initial fears whether your desire to carry on dating increases or stays closer to what your preference has been until now.
There's no reason why you couldn't decide "nope, I'm not into it" and then work on other stuff instead.
How has it been going with the therapist otherwise, have they been helpful in other areas?
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You