Because of the time frame of your depression, it is not necessarily related to your transplant. Unless you have had a change of medications in that time?
You have a lot on your plate, in short. You have dealt with more in your life than the average person your age, so you might even be feeling a little isolated.
There could be any number of reasons, environmental and biological, that are causing you to feel this way. I agree with others here about counselling and perhaps some anti depressant medication. I NEVER stop taking anti-depressants anymore, I have accepted them as part of my life as much as my immunosuppressives are. Start by talking to your doctor who can possibly prescribe antidepressants, or else refer you to a service such as a psychologist or psychiatrist. Most of all, DON'T BE SCARED to take this step. It's a big one, but you will never look back. You will be so glad you did.
Hang in there.