Parents and close family can be the hardest to budge on the issue. Some come around sooner than others. Some, sadly for them, never do. If your mum says she is supportive I suspect she doesn't really understand what being trans means for you. Have you discussed your intentions (be they HRT or transition, etc) with her? If you are still presenting mainly as "female" around the home she is likely to be confused. So perhaps another chat (or three) is in order. Coming out as trans is really a series of conversations and actions...it's much more complex than coming out as gay. It'll take time, she will probably become your strongest ally once it sinks in and she realises you are her son. Whenever people at work misgendered me (always accidental) I would simply as "who"? to let them know I didn't know who they were talking about. They then corrected themselves. It's a form of brain mapping and training for them!!