Taking more responsibilities than you have in your agenda builds skills. There comes a point though where enough is enough. When people are accustomed to your acceptance of their workload especially if they are in a higher position in your company, it is quite hard for them to accept the fact that you can no longer fulfill their obligations.
I received a call from a friend. Her boss was excellent net worker. But his workload was safely rested on her shoulders. She told me: I do all his work, he gets the credit and today I learnt that he is getting paid 10 times more than I do! For what? Chatting with the boss?”
Well, here is the thing; we need to add in our toolbox of skills being excellent net-workers and communicating our needs as well as learning how to toot our own horns. I am not very good on that either, but many people can excel to pretty much only the above skills and get to wherever they want. Yes, life is not fair, but we always have the choice to roll up our sleeves and work on these particular soft skills as well.
Next time someone tries to delegate their workload, you can always pinpoint yours. Tell them you would love to help them as always but that time you have your deadlines and you want to perform at your best. And then stop excusing. Smile. Go on. You are done.