well obviously i don't know your mom, but if she even knows what "trans" is (or at least seems to have some idea) that's worlds ahead of a lot of moms, so it sounds like she could be pretty sympathetic to your case. i'd just ease your folks- and yourself- into it a little by saying that you are "questioning" or "having some issues concerning gender" and want to see a therapist that specializes in gender. it takes a lot of people a long time to figure out this stuff, so don't be trippin' and being hard on yourself for not knowing exactly where you stand and what you want. i think its pretty common for a lot of people to have a "denial" phase when it comes to accepting themselves as "trans" or "genderqueer" or whatever, so i'd say just be honest with yourself and be like, "ok. i don't know right now, but i am figuring it out" and try to be open to what works and what doesn't.
generally it seems to me that anyone who devotes a lot of time to wondering if they are "trans" probably is... or, at least, is not cisgendered. cis people, generally, don't wonder if they are trans all the time (i'm sure maybe there are exceptions, but i think that would be fairly unusual). the next step is sort of figuring out how you want to deal with it (i.e. name, pronouns, transition, etc). good luck.
