@lauraloo81, I did have a feeling of guilt after my transplant. I realized it immediately after surgery.
I can remember seeing smiles on the faces of my husband and sons as the entered my hospital room after my surgery, and I could see the lightness in their step. Best of all was the ability to laugh together again! But - I also remember how, in an instant the mood changed as tears filled my eyes and I thought of another family, my donor's family, together in a completely different setting.
It took me years to learn that I was not alone with that feeling.

I also felt a guilt/regret because I (60 yrs old) got my transplant while someone else younger that I did not. Common sense about correct match did not enter my mind at that time.

Here are some discussions where members do talk about their own feelings of sadness/guilt after their transplants.
Your gratitude changes because of organ failure...
https://connect.mayoclinic.org/discussion/your-gratitude-changes-becuase-of-organ-faliure/
Writing to Your Donor's Family.
https://connect.mayoclinic.org/discussion/writing-to-your-donors-family/
Laura, Is this the kind of regret that you mean? If you are comfortable to do so, would you share how you feel?