I'm sorry Ersi, this stuff is hard. I can understand why your heart is broken, this is your first born daughter.
I can understand your sorrow. As you said, your head knows what to do but your heart isn't buying it.
It takes time for us to let go of our kids. Your daughter made the choice to move out at 18 and cut a lot of ties with you....... and that isn't the usual trajectory nor what we expect or are ready for. And yet, it is what it is.
You have therapy, you're reading books and you''re feeling your feelings, for a little while that is how it's going to be until you emerge out of the process of letting go and detaching and accepting what is.
It isn't easy, but, like the rest of us here, you have no control over what your adult child is doing. All that is left for us to do is to learn to accept it. And, it AIN'T easy.........in fact, it may be the hardest thing you'll ever do, it was for me.
While you're going through this transition, be VERY VERY kind to yourself. Allow yourself to be sad and to cry, to not understand and to rail against what is........you're in a grieving process going through the stages of grief........... which are denial, bargaining, anger, depression and then acceptance. Not necessarily in that order. It takes time. This is a big transition for both of you. She is apparently needing to test her wings independent of you. All you can do is let go and allow it, you have no other choice.
I'm sorry, I know how much it hurts. Hang in there, down the road a piece it is going to be better. Take it one day at a time.
Sending hugs Ersi........