If you don’t want to say no to a project, say “not right now” and schedule it for the future. It’s hard to turn down good opportunities, but often they can wait until the time is right.
Health is often the first thing to go, and it shouldn’t be. If you start thinking of your health as not important, you’ll find it hard to recover from the burnout.
Perhaps overwhelm has to get bad for you to better learn how to scope projects and manage your time in the future. It’s a hard skill to learn and when you make mistakes, it’s an opportunity to remember to allow more time for the next project.
Getting through an overwhelming time is a lot about mindset. You have no brain space for being worried about stress. You need it all for working on the projects and getting them done. So write down what you have to do and just start doing it, step by step.