If he thinks you have "bipolar tendencies," I hope you are on a mood stabilizer in addition to the zoloft. Lithium is cheap - it is a naturally occurring salt. Depakote has been generic for awhile, but you would have to price that. Lamictal I think is now generic too, but not as long, so again, would have to price that. If you are tolerating the medication well, there is no reason to stop it. If the side effects are really upsetting, well, then you may want to risk a slow, slow taper and see what happens. Does depression run in your family? If so, bipolar is more likely - it's very genetic. Atypical depression, where you sleep a lot more than usual, instead of insomnia, is a bipolar sign. Agitated depressions, where you pace around and feel on fire - that's another bipolar type thing.
go to bipolarworld.net and click thru to dr. phelps (ask the doctor). He has put a TON of medical information on his website, and even has a book you can read for free on bipolar II, "soft" bipolar, which is probably what you have if you have bipolar. He has lots of tips of things that may help that cost nothing, too, like exercise, keep your sleep schedule the same every day. Light boxes help people who get depressed in the winter more, and are cheap compared to most meds ($300, but only a one time purchase).
I've had bipolar for many years