Sick note information needed for UC

I went to see a GP today about claiming universal credit. I told her that I wanted it documenting on my sick note that being found fit to work would have a detrimental impact on my physical and mental health, as that's legitimate and I have very poor health. She said "we can't do that. All we can do is tick the box where it says unfit for work". And then she just wrote "anxiety and depression" as the reason. If I was from the DWP and I read that, it wouldn't sound serious to me at all, when my health is actually in a terrible state. I have terrible chronic mental health issues from lots of abuse, I'm traumatised with severe PTSD and none of that seemed to get through to the Dr. On top of that, I have a number of limiting physical health conditions that cause me severe daily pain. There is absolutely no way I can go through any sort of questioning that would make me go into detail about why I believe I'm not well enough to work. I'm in a very fragile state with my health. I feel terrible all the time. Should my GP be putting more than just what she wrote in order for them to take me seriously? Any advice please? Thank you.