I feel very strange almost all the time, but it's difficult for me to pinpoint how and why.
I suspect a mental illness because my behaviour and thoughts aren't normal to me. I have felt this way for ...
Hon, I think part of your problem is your continual efforts to self diagnose by matching your symptoms with what you read about online. That being said, however, I do agree that you seem to be having some mental health issues that need to be addressed.
Is there a reason why you seem to think your parents would keep you from getting help from a qualified professional?
If you don't feel as though you can ask your parents to arrange for you to get mental health counseling, then try this: Ask for an appointment for a check-up with your regular doctor. During the appointment, confide in your doctor and ask him or her for help. S/he will either know what to do or will be able to figure out how you can get help for your problems; furthermore, should notifying your parents become necessary then a professional opinion might carry more weight than a simple request from you.
If not that, then try asking another trusted adult (family member, spiritual leader, friend's parent, school counselor, teacher) for help. You cannot wait until you're older, you need it now.
Good luck, sweetie.