@katgob, Having a bone marrow transplant (stem cell transplant) is getting a second chance with life. Your defective immune system/bone marrow will be completely replaced with that of your donor via an infusion of cells with the goal of new, healthy bone marrow capable of making healthy blood products again.

From what you’ve said, your genetics testing has been done already so now your doctor has launched the process to match you to a donor. Depending on your nationality, it can be pretty quick or take some time to find a donor who matches your HLA stats.

The rest can move fairly quickly after that.
Some things for you to know about a bone marrow transplant is that most clinics which do the procedure require you and a caregiver to live within 30 minutes of the clinic for about 100 days. Each clinic can have their own protocols and requirements. Mayo-Rochester, where I had my transplant was all out patient. I stayed nearby in a hotel for 4 months with my husband. Other places will have you residing in the hospital for several weeks then convalescing nearby. But you will be required to have a caregiver with you.
As you draw nearer your procedure, there are several of us in the forum who have gone through this. We’re here to share our experiences, answer any questions you have and to offer hope. I’m at the 4.5 year post transplant mark and amazingly healthy! I feel as though nothing ever happened. It’s not an easy journey, especially the first couple of months. But you’ve already gone through treatments for breast cancer so you know you can get through this!!
Let me know if you have any specific questions, ok? Hugs.