If you have the ability to make up the difference, it would (in general) make more sense to sell now and eat the negative equity. Otherwise you are just chasing depreciation with more monthly payments.
BUT I would also say that if it meets your needs, you know the maintenance history, it's a solid vehicle, and the depreciation curve is gradually flattening out, I might be inclined to consider keeping it and just doubling up on payments. I have played the downgrade game before and not always been at a net gain. I've bought beaters "cheap" (3-4k) and then spent 3-4k just getting them to a minimum level of reliability (and this is doing the work myself). So lets say your Ridgeline got 25 MPG and had 30k miles, and was worth 14k, would I downgrade to an Accord that got 28 MPG and had 100k miles? Probably not. Big picture, including total cost of ownership, the difference would likely be less than the difference in payments. Just food for thought. Of course if you could go no car / zip car / cheap beater without fixing it up, I would think that you would definitely have a net gain by just getting rid of the Ridgeline ASAP.