ATG is an immunosuppressive therapy that's very common for SAA treatment. However, you may not have had ATG since you had your transplant so soon after your diagnosis. It seems to me that there wouldn't have been time to have ATG treatment and see if it worked. My guess is that you went straight to transplant instead. To be sure, you could ask your parents or get the old treatment records from the hospital.
Your use of prednisone and cyclosporin was brief compared to many other patients. Some of them are on these drugs for many years. Your exposure to them certainly affected you at the time (growth, hair) but there may be very few long-term consequences or none at all.
You asked if you should worry about this. The answer is that we don't know what the future holds for us, as Karenish pointed out, and we can't do anything about the past, so there is little to be gained by worrying. But I still think it's worth understanding what happened to you as a teenager.