Start with asking yourself why you are agreeing to the projects. Is it that you want to do them, or are you being pressured?

If it’s you, then it’s about clarity. If it’s them, then it’s also about clarity.

You need to make your workload clearer to yourself. How can you even know if you have capacity to take on a new project?

You could simply list your projects and commitments / deadlines. Sometimes the list is enough.

You could block out your time, which is your actual resource.

If that’s failing, you could try better estimation, but it’s extremely difficult.

I would argue for simply having a maximum number of spinning plates. Like, two.