How do I help people with severe or complex disabilities?

You have built real skill. Most situations you can adapt now, and many people are better off for it. What remains is the hardest ground, the severe and complex cases, and it is honest to admit they still stump you. They would stump anyone who takes them seriously. These cases are hard because each one is truly unique. There is no standard answer to reach for. That is not a hole in your training. It is the nature of the work, where creativity matters more than any rulebook and every person needs something built just for them. When you hit a case you cannot crack, bring in others. Ask a colleague, call a specialist, learn from someone who has faced it before. The best in this field are not the ones who know everything. They are the ones who keep asking. Let one hard case teach you something you can carry to the next.