Hey man, I hate dumb comments like that. My in law said to me the other day that I "put the 'his' in 'hysterectomy," while I was openly talking about anxiety I was experiencing. I was upset for 2 days and finally talked about it today, stating that it is never funny to make light of a situation that truly carries so much weight. He replied saying he thought it would be okay, since "maybe I was at a turning point in the whole process." As if trans jokes would become funny at some point because I had a surgery.
This was coming from someone who literally had to drag me into my house on multiple occasions, when I was black out drunk and constantly talking of suicide. That hurts, because after knowing someone for more than half your life you'd expect they might have a little more understanding and/or sensitivity when it comes to your situation.
Some folks will just never get it, and it is particularly painful when you are family or live with the person. The more time that passes I find it's easier to just let go of connections, in your mind if you can't detach from a physical stand point. After years of dealing with the same crap, this time I've decided to stop intentionally interacting. It's not easy but will have a greater impact on my mental health.
By detaching from care, you can take away that person's power. At least to a degree. Certain words will always hurt, but when you take away the credibility from that person in your mind, they will hurt less over time.
Sorry this turned into a long post, that wasn't intentional. I'm sorry your mom said that to you. Stay strong, and know that you are strong, and eventually things will get easier. You also have a whole band of brothers and sisters here to share with that understand exactly what you are going through.