Don't worry about it. And trust me - I was in a similar position.
I transitioned whilst I was still at home when I was 22, with two brothers the exact same age as me (I'm a triplet), so keeping them unaware was
hard. But in the end they were accepting - they knew that at the time, I was depressed, so whatever made me happy received their support. Hopefully your mum and your flatmates will think similarly.
I had never even contemplated any of that stuff too, but in hindsight, it would have kept me occupied and also gave me a better starting point.
But there comes time when you have to take action and take control of your own life. You're right - "maybe" isn't good enough. Even though its daunting, sometimes you have to take a leap of faith and just go for it.
Can I ask, how old are you? If you're of age, then you shouldn't have a problem securing a therapist on your own, without your mother's intervention.
Also, don't worry about offending me! And, if you want, feel free to PM me

I'm more than happy to talk to you privately!
X