Hello, I have some questions, I want to know if they are grammatically correct

1. Should I say "I understand totally", or "I totally understand"?
2. If I concern about a condition, do I say "The condition concerns me", "I am very concerned about the condition", or "I am concerning about the condition"? Or are they all correct?
3. "I feel disgusted" and "I feel disgusting" have different meaning right? But are they grammatically correct?
4. Are both "I am very worried about it" and "I worry about it" correct sentences?
Thank you very much for taking the time to help me
