Welcome here to the Forum. I'm sorry your circumstances are so bad and that you have to try to fix them by yourself.
Look, I'll have to be blunt, you asked "Not sure what to do". I am not sure you can do anything. 3 years together is a long time and you would think you know the other person by then. I'm sure you know yourself.
I'm not so sure your ex-partner does know herself, and has been in the process of finding out. Living in partnership with you must have been good -or at least comfortable, however asking for a ring may have been as much trying to find out about herself as anything else. Wondering if a symbol of permanence would make her feel permanent too.
It does not seem to have worked , having received a ring and not felt she wanted to continue with you may well have been part of her slow self-discovery. No reflection on you, just she is finding she is not the person she thought she was when she entered the relationship
The fact the relationship stopped may well have made her stop believing in love and still be unsure about herself and you too.
I'll admit I'm guessing, if you think of a more likely explanation please sing out, however you have not mentioned any drastic changes in life, such as another person, or even pressure to be together full time or marry. As a result I'm tending to thing she is discovering herself as she grows and the years pass.
What to do? I'm not sure. I think I'd give it time -but no pressure, and see what happens. She may finds she needs you, she may find otherwise.
Perhaps set yourself a date, and if nothing has happens by then start on a new social life. I know that might go against hope and instinct, but is a realistic way to deal with a most unhappy situation.
Please say what you think, we are here for you