Welcome to the Forum. I can see that your constant changes of mood would make life very difficult indeed and without a specific cause to point to cause a lot of anxiety too, after all it is a most unusual way to be and as you already know does need investigating properly.
Can I offer a couple of suggestions?
In the short term being able to talk or web-chat with someone that understands can be a real comfort, Kids Help Line (1800 55 1800) will allow you to contact them more than once and are very experienced and realistic.
The second thing is to get your GP to understand how much this is affecting you life, so they get the idea some form of action, rather than just giving generic advice, is needed. To do this I guess having your mother with you is very counter-productive and realy you need a one-to-one with the doctor by yourself. Persuading her might be difficult, but if you can get your doctor on side it should be doable. Maybe she can come in later
So how do you let your doctor get a true picture? I found I was not able to explain face to face many times -and that was with no third party there. My solution was to take a few days before a pre-booked long consultation and write out in point form all that was happening to me. This can take a fair bit of effort and I had to summarize, otherwise it would have been as long as a daily journal.
This worked well, I had time to make sure everything was included beforehand, and the doctor had a listing to work from. I ended up just answering questions or expanding on what I"d written.
I guess you could put at the top of the list you needed to speak to the doctor alone.
Then see what happens, hopefully the doctor will take it all seriously and try to deal with the whole problem.
Would you like to come back and talk some more about this?