Just sell for less. If everyone else demands high valuations, it's easier to sell, not harder.
Your investors gave you $50 million at a $500 million valuation. Google wants to acquire you for $100 million. Your investors may not let you.
Or you can cancel some of your own shares and take the $50M.
Or if you still have runway and believe in the business, keep going and raise that $100 million.
It sounds like the problem of having scaling issues: it's a problem, sure, but it means you're already a success. If you never got that $50 million to begin with because valuations weren't high, then you might be a failure now instead.
Be sure to check your term sheet. It could be that your investors have a liquidation preference that includes a multiple. Based on my (admittedly limited) understanding, if your investors gave you $50 million for 10% of the company, with a 2X multiple liquidation preference, they would be entitled to the first $100 million of any sale, plus 10%. So you may be able to sell for $100 million, but you wouldn't see a dime.