Oh dear, it sounds like you are stuck with it. All you can hope for is that the value increases in the future but that's why investments always come with a warning that values can go down as well as up.
But now that you've paid off your other debts, can you afford to convert the BTL to repayment, to hopefully get it repaid before the end of the term? If the property is a desirable rental, at least you should be able to continue renting it out for the rest of your lives if necessary to bring in an income.
You will need to keep an eye on the interest rates of your borrowings, both the BTL and your main mortgage, to make sure they're on the best rate possible. As the BTL is in negative equity you're probably stuck with whatever product you're on now, but there might be the option to remortgage your main home if it qualifies for a better product and use the money released to pay down the BTL, if the interest rate is higher.
As an aside, if your DH is self employed, has he applied for the coronavirus grants that he should be entitled to? There's one running now, and a second portion that will be available soon.
And even though it's far from the right time, and your confidence is low, you looking for work might be necessary. Is there any chance of any of the temping contracts you have done opening doors to permanent positions?