It has been 18 months since I worn the ring. (Does she still wear the ring or not? How to express she is still wearing it?)
I have been wearing the ring for 18 months.
The sentences above answers your question as to how to express that she is still wearing the ring.
It has been 18 months since I started wearing this ring could be an option too. Mind that the sentence "It has been 18 months since I worn the ring" is incorrect. First of all, worn is the past participle (the Past Simple form is wore). But "...since she wore the ring" wouldn't be correct either. She's still wearing it. You need a verb that would mean the action which is in the past. For example, "start".