Perhaps more importantly than a mental model for thinking about this you might need the right mental state. You're tired and not seeing a path forward. That leads to burnout fast and isn't a great space to make big decisions from.

I'd talk to your co-founder and start by taking some time off. I bet for the past 3 years of grinding on the startup you haven't done a good job with vacations. If you went and fully unplugged for a month (or more) you might be in a place for a better perspective.

You might feel like you can't get away - that you're too essential to some work right now - but in my experience a top role of co-founders is to keep the passion and vision strong. Where you sound like you're at mentally you may be doing more team harm than good and your co-founder may be surprisingly supportive of you taking some time to get rest.