Technically you are married (even if psychologically or socially you are single), so if you want to marry someone else, you should first get a divorce, and then observe iddah if necessary.
Istikhara is not appropriate for a thing which is inherently forbidden (and to marry someone else while you are married is forbidden).
This is why it is not a good idea to let a nikah without an actual marriage hang for a long time, whether it is as an engagement or after marriage. Of course it happens and I am not saying it is your fault personally, as usually it is the fault of society, but I am just saying it is not a healthy situation.
Anyway I hope you can resolve your marital situation soon (or, rather, dissolve it, if that is your intent).