As the great Brené Brown says, "boundaries are simply our lists of what's okay and what's not okay. When we fail to set boundaries and hold people accountable, we feel used and mistreated. This is why we sometimes attack who they are, which is far more hurtful than addressing a behavior or choice."
So - stop saying yes when you want to say no. It is perfectly okay to say "I can't take that on right now" or "My plate is full."
As for your personal relationships, you need to be very clear about what is okay and what is not okay. If people don't respect them, they need to be held accountable. It's VERY important to mean what you say. If someone crosses your very clear boundaries, you must hold them accountable. Boundaries aren't set until you follow through on the consequence and stick to your convictions.