Trapped and Stressed by Work Responsibilities

I work at a consulting company I've been getting more and more stressed at work lately due to a bunch of factors:

• "slow" year overall for the business despite me working on 3 projects at the same time, so the bosses are stressed and unhappy
• company is extremely small (~10 people) so there's no way to unload responsibility and if anyone gets sick the rest of us have to pick up the slack
• all 3 of the projects I'm working on I'm the lead/only active employee on and my boss doesn't do or want to do anything to help lighten the workload, he also continues to say yes everything the clients ask which I ultimately become responsible for
• my family is deeply involved in the company and I've known the other employees and managers for over 15 years, so I don't feel like I can just walk away
• I have voiced my anxiety about juggling these projects but my boss' response was basically "I know it sucks but you gotta deal with it"
• I feel like if I quit I'll push the company over the edge, but if I stay I'll burn out.

I don't really know how to navigate this other than just putting my head down and trying my best until we reach a crisis where project deadlines can't be met because I have too much going on at once.