I have been separated now for 21 months. Would it be ok for me to start seeing other women?
If you are not legally divorced, then no it is not okay. According to some state laws, the divorce must be submitted for a period of time before it is official. Even then some states require a period of time before another marriage license can be issued. It can vary from a month or two to almost a year.
If you are not legally divorced, but only separated,then in the eyes of the Church, you are still very much married, and are required to maintain fidelity. Thus dating while separated is not acceptable. Separation is a time given for couples to work out differences on their own without ending the marriage. I would encourage you to make every serious effort to do so. If divorce seems the only resolution, then complete the required steps to obtain the divorce before you start dating again.