I was the second worse in my whole skool at mafs. Only beaten by the skool cat and it was stuffed.
I had no problems with the ATPL ground school or exams. I spent a great deal of time practicing questions and trying to understand what the questions were trying to ask, then unbelievably I could answer very complex questions in stages that started to make sense.
I think that being quick witted and an ability to learn is the key, backed up by an ability to mentally break down a question so that you get a ball park answer, before going deep into the question.
Give it a go. The ATPL questions are glorified PPL questions.
Don't stress.