You seem to have left out one key piece of information: Do you believe in the product and the management team? It is not at all uncommon for founders (and early key players) to have salary delayed during initial funding.
Start-ups carry MUCH greater risk, and often greater rewards. These potential rewards include access to discussions that if held in a big company you might not be senior enough to participate in. Another potential reward is getting to do a far broader range of tasks so you expand your horizons and your experience.
If you went to a start-up for stability, then you made the wrong choice.
If you can not have a candid discussion with the CEO, you may want to leave.
If the dollar amount has become too much to risk on a verbal promise, it is not unreasonable for you to request that the company issue you a promissory note for your earned salary (not future salary), and they should be willing to make a "personal guarantee" - so they are on the hook as individuals. IF there were assets in the company (doubtful as you describe things) you could have a note for your accrued salary be secured with something called a "UCC-1" - ask your legal counsel if you think there are assets in the company that can be used for collateral (security).
Being part of a start-up means you are willing to do the unconventional to solve problems which might otherwise be solved by throwing money at the issue. Find a solution or decide that start-up life is not your cup of tea. For most it is not, but for many there is nothing better in the world. Know yourself, then make the best decision for you.