Of course this feels like abandonment and it's critical that you work through this transition with your T to alleviate that as much as possible. It does feel that way and you need to honor those feelings by expressing them the way you are, but keeping in the security of the backdrop that she really is NOT abandoning you and that she is making sure to find someone who will care for your needs when she is no longer able to. Perhaps the way a mother would who finds she is terminally ill and needs to find someone to care for her child when she is gone. In this case your T is not dying and she has offered some contact with you in the future, but this is a tremendously painful shift taking place right now and she and your new T will make this transition work out as best as they can. I am sorry that it can't be painless.
I know this hurts so keep posting. You are really strong and handling this much better than it may feel like you are. As hard as this is I get the sense that your T is doing all she can for you and that she cares very deeply for you. That is something you can hold onto forever and nothing can ever take away the meaningfulness of this beautiful relationship you have had.
A flower may fade and whither as it slowly loses it's pedals, but that does not mean it was never beautiful to the bearer and that it never brought joy to the beholder. -JM