Hi anon – I would say, with all due respect, so don’t take this the wrong way, but you are in therapy FOR YOU. To sort your issues out. If you start worrying about whether you should or shouldn’t be telling things to your mom, or whoever, then that is just going to cause you more problems – on one hand, your sorting yourself out by going to therapy, but yet, on the other hand, your giving yourself more problems because your worrying about things related to therapy.
You could just try explaining to your mum, that even though you are close, exactly what you’re feeling right now… i.e. that you aren’t ready to talk about what is going on in therapy or what you’re being asked to do. Yes, all she probably wants to do is help, but sometimes people just need to be left along to sort things out for themselves. Try telling you mum that as nicely as possible, and I’m sure she’ll understand. I don’t like it when I think my best friend holds things back from me… but I figure that sometimes you just have to leave people alone for a bit, and eventually they’ll come back to you when they are ready. And that usually works. That might be what your mum needs to do with you.
If you don’t want to say that to your mum, in case she gets hurt, you could always (I know this isn’t healthy) go with denial… of course I’m not holding anything back from you mum… I have told you everything. That might make her feel better, but how would you feel after lying to her?
At the end of the day, it comes back to the one point I made before: only do what you’re comfortable with. If you try to do something that you’re not comfortable with, then its just going to make you unhappy and therapy might end up curing one problem, but giving you another.
EDIT: If you are finding it difficult having a boundary, try going slowly. Tell you're mom a bit more each week, or after each session and when you find yourself uncomfortable, or that you're in a position when you don't want to say anymore, then STOP.