Hello.
You must be having a hard time dealing with anxiety thoughts and feelings. As you have mentioned that you have been having these feelings for a while, and you are unaware of the cause. It could be that a certain event from years ago could have triggered the anxiety and other feelings. This cannot be concluded, there may be certain other factor to it as well.
Next Steps
I would encourage you to meet a counselor to address about the same. With a counselor you would be able to share your feelings and thoughts as they are which may help in understanding the real cause. It will also ease your mind when you share your thoughts and feelings.
Health Tips
It is important that you do not keep these feelings to yourself, try to share your thoughts to someone whom you feel close to and trust. I understand that the process is tough, however in time you will see good results.