Many of us say yes automatically — then pay for it in evenings and weekends.
A workable 'no' has three parts: acknowledgement ('I can see this matters'), a clear boundary ('I can't take this on this week'), and where possible an alternative ('I could look at it Thursday, or Priya may have capacity').
Start with low-stakes situations to build the muscle. Discomfort when setting a boundary is normal and fades faster than resentment does.