Limited capacity turns boundaries into mathematics rather than attitude.

Boundaries get discussed as though they were a matter of confidence or attitude, and mostly they are arithmetic. There is a fixed number of hours and a fixed amount of energy, and when the requests exceed the supply something has to be declined regardless of how you feel about declining. Framing it as arithmetic removes a lot of the guilt.

When you next need to refuse something, do the sum first. Hours available, hours committed. Then say no as a matter of capacity rather than of preference.