Sitting On Your Hands Is Never Part of Your Job

I submitted some work to a major U.S. publisher almost a year ago. A few months later, I received a personalized, lengthy, warm email from an editor who indicated she liked my work but that she wasn't ready to take on the project at that point. She did suggest some changes and invited me to send it back to her. I revised and re-submitted. A few months went by and I emailed her again to check the status. She replied that she hadn't yet had a chance to get to it. A few months later, I emailed again. She wrote back: "I need more time to review this submission." And it's been a while since then...How do I interpret this? Is my story just collecting dust under her desk when I could be actively sending my work elsewhere? 

YES.

 Or might this mean that there may be some genuine interest and that I should just be patient?

YES.

I don't know how many times I've gotten variations on this question.  DO be patient.  And DO keep submitting elsewhere.  The changes that she asked for should be exclusive to her... for maybe six months, to be generous.  Until then, keep submitting the previous draft elsewhere.  After that, submit the revision (assuming you think it's stronger) elsewhere.

Editors can take a goddamn long time to just LOOK at something, at which point they may be very excited about it.  An interminable wait may not mean that nothing's ever going to happen.  But you should NOT be waiting for that day (if it ever comes) to KEEP SUBMITTING.  

KEEP SUBMITTING.