OK dude, first off - CHILL!
I'm assuming you're seeing a local psych rather than having an appointment at the GIC? Are you looking to be referred to Charing Cross in London, or will you be sent to Leeds or one of the other GICs, do you know?
If it's CX, the psych you'll be seeing is just there to make sure you're suitable to be referred to CX, rather than decide whether you should or shouldn't be given T and surgery referrals etc. It's the job of the CX psychs to "diagnose" you, not the local psych - the local psych should just be checking up that you're not wasting time, or suffering from any mental health problems that might be confused with trans issues.
I'm not sure of the procedure for Leeds or anywhere else, but I can try and find out if that's where you're likely to be going.
The fact that you're doing ok at the moment won't make them try to deny you T - in fact, it'll just prove to them that living as male is the right thing for you to do; that's pretty much the point of real life experience!
Oh, and if you want your mum to go with you to the appointment I don't see why you shouldn't take her - lots of people take someone for support, and at leasst it'll show that you have the support of your family!