If your problem is not the sheer quantity of work, but the way it is organised and allocated to you, the issue lies with your manager or the system that he or she has to work within. A common mistake among managers is concentrating on the parts of their job that they do directly, and neglecting to oversee the work of those they manage. Of course there are some jobs where you are simply responding to external events or crises that cannot be planned. But even here there should still be sufficient cover to cope with all but truly exceptional events.
If your manager is consciously or unconsciously making life difficult for you, you may need to raise the problem directly with them in private. If there are some better ways of organising work, then suggest changes in a way that doesn't threaten their authority. Or suggest a working party or time at a team meeting to talk through the issue.
