There is nothing selfish about doing what you have to do to take care of your physical, mental and emotional health.
Your mom is entitled to mourn the loss of the man she thought you were, but she is not entitled to claim property rights in that identity and force you to stay in it.
She is seemingly ignoring the fact that you are her daughter, and that you will emerge from therapy a better person in every way. Hopefully if she gets some therapy or support herself, she can learn to appreciate the person you truly are rather than the one she thought you were.