I'm Dr. Jackie, an interpersonal communication researcher/professor and relationship expert. And I want to help.
First, I am rather shocked that you have said your friend is a psychiatrist. A person in the medical and behavioral sciences fields knows how damaging such words can be. So I am sorry she said that. Sometimes even professionals forget all their training when it gets personal.
Second, relationships do grow and change all the time. But usually the changes are subtle, so we don't realize the small changes....until one day we wake up and "THINK" that change has occurred overnight. Of course this is not true. And your own admission is that they have been seeing one another for a long time.
I am not sure how frank you have been with her, but you need to be. You need to tell her what you are feeling...even though you know it's not logical. Given her profession, she is extremely well versed in dealing with scenarios when emotions take over the place of logic. So tell her you know it's not logical, but it's how you feel. She probably knows you need to see a therapist. Counseling your working through these feelings would be my recommendation. But medically, she might be able to better direct you to your medical doctor, who may put you on a different antidepressant or add to it. When we have chemical imbalances, sometimes medicines become ineffective after awhile and need to be increased, changed, or perhaps added to.
Please talk to her and be open and honest and be willing to accept her medication recommendations. Also, see if you can do a girls' night out once a week on a certain day or maybe a regular lunch on a certain day each week. The night out probably would be better. I am guessing that given your long, long term friendship, that this will be a good thing for the two of you. :)
Please let me know what you think. Best,