It sounds wonderful that you are such a supportive friend to her but friendships are about you BOTH making each other feel the best you possibly can and picking each other up and if it is constantly one sided, understandably its going to take its toll and the nature of the friendship is going to have changed. Your GCSE's are super important and you mustn't sacrifice those. Obviously it depends on the nature of these problems she struggles with but there are self help resources out there as well that would mean she doesn't have to talk to someone like childline to make it accessible in an easier way to her. I will try and dig some of those out if you think it will be worth it. For now I would try and say have that time with her where you try and make her feel better but maybe try not to let all of it become about constantly hashing out all her struggles. Talking about it is super important don't get me wrong but if she can try and find something positive and enjoyable that kinda takes her mind off of it but just kinda brings her entire mood up. As for you I would say have that time with her but when your away from her and spending time with family or whatever, save some of that time for yourself if possible, where your mind is set free a little bit and maybe avoid these platforms which you associate with her. It sounds like quite a difficult situation and completely understandably these people without meaning to can be quite a draining presence and what your having to do is very mature and a lot to ask for someone of your age I'm guessing.