Let me tell you, it's taking me a long time to understand this concept I just have a hard time saying no when I needed time for myself.
Understand your limits and be honest about them. Don't be afraid to say no when it's necessary. Find ways to politely decline requests that you cannot fulfill.
Focus on the tasks you can accomplish and prioritize accordingly.
Make sure you are taking care of yourself in the process. Take breaks when needed and don't overextend yourself.