[JoachimSchipper]

Consider (diplomatically!) talking to your boss.

If the company is successful-but-not-spectacularly-successful for a long time, one way to handle such issues is by structuring the flow of money from the company to the founders as dividends - that way, you'll get paid some fraction of what they get paid, indefinitely. Note that there are other ways to structure that particular money flow, though, and there's very little you can do to influence the choice.

For a select few companies, secondary markets exist that trade in illiquid stock. I understand that many equity agreements forbid selling on those markets and/or quickly selling on those markets. However, this only works if the buyers have a reason to believe that the stock will ever be worth anything at all.

You mention valuations, which suggest that some fairly savvy people have decided to exchange money for shares in this company. Do you know how the investors expected to get paid back? It's possible for founders to screw investors, of course, but this may be another thing worth looking into.