Hello and welcome to the forum. I am very glad you came here to chat. Everyone has their own story but we can relate to each other because we know how difficult it can be to open up to anyone.
Please know you are not a weak person because you are depressed. It takes strength to say you have these feelings and not struggle on by yourself. One suggestion I have to start the ball rolling is to download and print some information about depression and give it to your mom to read. Tell mom you want to talk about it when she has read the fact sheet. Go to The Facts above and navigate to the information you need or go to Get Support and navigate to the information pages. Read them yourself. It's always good to have solid information about your mental health.
Why not suggest to mom that you both go out for coffee or shopping or anything else that gets you both out of the house without your brother. Say you want to talk privately.
How did your meeting with the psychologist go on Monday? Do you feel he/she understood you? We are all good at hiding our feelings because it can be scary out there. Community attitudes towards mental illness are getting better but it's still hard to talk to someone about it. If you think it would be helpful write some dot points listing how you feel, how you went to Headspace and that you are going to a psychologist.
It will be a lot for your mom to take in I imagine so do not be concerned if she does not ask many questions or talk much about it. Mom may want to think about what you have told her before going much further with the discussion. You can ask her not to talk about it in front of your brother. Organise another time to catch up with her.
I see Sophie has suggested you contact the Kids Helpline. Their web address is www.kidshelpline.com.au and their 24/7 phone number is 1800 55 1800. And of course you can contact Beyond Blue or Lifeline. All good services where you can talk about your concerns.
Please come and chat here as much as you want. It's a good place and of course is anonymous.