I’m so sorry you are in such pain and that you are feeling so confused and alone.
It may be easier to ask your mom to take you for a checkup with your pediatrician than to ask her to get you a therapy appointment. There are many medical problems that can make a person feel depressed and hopeless. You want to be sure that a medical issue isn’t being overlooked. At 14, you can certainly ask the doctor to give you a few minutes in private – without your mom – to talk about personal issues. Most doctors will respect that and will ask your mother to step outside.
If you and your doctor agree that you are physically okay, your doctor may refer you to a counselor for an evaluation. Your doctor will be your ally with your mom. An hour with a counselor may help you sort out whether you are going through usual, though painful, teenage emotions or if you actually meet the criteria for a mental health diagnosis. Either way, the counselor will make some suggestions about what you can do to feel better.
I wish you well.