You're better than those who are trying to tear you down. They are ignorant, and this ignorance often turns into fear and hatred. It's one of those "easier said than done" things, but it holds true for anyone that you can't let other people's baggage weigh you down. You aren't worthless, and don't let anyone make you think you are.
And gender dysphoria is most certainly not a mental disorder. Those who compare it to things like anorexia nervosa do not stop to consider that if someone is allowed to "be" anorexic then they will suffer, cause irrepable damage to their body, and it could kill them. People who say it is a body dysmorphic disorder do not realize that those with that condition typically worsen when they get any sort of plastic surgery or other body modifications. And of course it's not delusional thinking as there is no real way to define gender in any consistent manner except through ones own sense of gender identity.
It does get a bit tricky and complicated as gender dysphoria does cause distress, depression, anxiety, social impairment, and other things that do typically indicate a disorder, but with medical treatment and transitioning transsexuals mostly improve in their well-being, functioning, and quality of life. Personally, I think "syndrome" is more accurate to describe it than "disorder," given there often is things such as depression and anxiety caused over dysphoria. But even that may not be accurate, especially as society evolves and becomes more tolerant and accepting, and more transsexuals are allowed to transition at a younger age, which theoretically should dramatically reduce the negative things like depression.
Probably it's best to just use the word "transsexual," as that is what it is. Really, it doesn't even need words such as "disorder" or "syndrome" attached to it, because it really is nothing more than a natural expression and identity that falls outside of our societies black-and-white gender binary, which is the thing that is actually screwed up.