Hi,
It sounds like ds is being affected by the meds too, which would make me want to stop bf or stop the meds. My biggest concern would be that the medication was affecting ds. If the meds are really helping you feel better, then I think you should continue with them since one's mental/emotional well-being can affect how they parent and interact with their child. However, if you aren't seeing any improvement, then I would try to get off them and continue bf.
You have given your son 8 wonderful months of bf and you should be proud of yourself for your commitment! I think it would be less 'harmful' to start on formula than it would be to expose ds to the Celexa through the bm. Since you want to start TTC, it might be time anyways, as you said.