I don't have the perfect solution to this and I hope that my response doesn't seem too harsh. The fact is that life has to go on with or without ITP. I know it's hard and you don't want to see your child struggle, but for her own sake, she has to take the bull by the horns and keep going.
I am on the PDSA Scholarship Committee and I have read many essays over the years written by teens with ITP. Most of them spoke of the struggles with fatigue and how it took them down for a while. But all of those teens decided that their futures were worth fighting for and they worked harder than ever to prove to themselves and others that they could do it. They pushed themselves through it and made many achievements. None of it was easy.
I understand how depression can grip someone with a diagnosis. Been there. There is help available. Sometimes giving in and not allowing yourself to live can add to the depression and feelings of failure, which is why it's even more important to keep trying.
She is almost an adult which makes it even more important. When adults are diagnosed with ITP, many have children and full time jobs. Due to financial obligations, they have to keep working because there is no choice. They also have no choice but to keep doing whatever it takes for their children, regardless of fatigue. Granted, it is a very real struggle (been there myself), but that is just the harsh reality of it.
I understand that you cannot physically force your daughter out of bed. But there are ways to convince her that her life has purpose and she needs to fulfill that for herself. Maybe counseling would help.
What are her counts? Has she had any treatments lately? Most find that once counts are at an acceptable level (over 50k), the fatigue is not as debilitating.