Hi...
From what you say in your post, I think you really need to open up to your therapist. You need to see a pdoc, I think, but it sounds like your mom wouldn't be open to that at this point.
Since your mom is on your b___ your therapist is really the only answer, maybe your family doc. If one of them thinks you need further help then perhaps they can convince your mom into letting you have an initial consultation.
Even though you are only 15, you have to try to take responsibility for your treatment. If you do have bipolar, you will have to that for the rest of your life. (I know that sounds scary, but you can do it!)
If you EVER feel like you might try to hurt yourself, go IMMEDIATELY to an emergency room. They will get you the help you need.
We all hope for the best for you, and we are here to help, or just to listen and give you a hug. Please take care of yourself, we are with you!