I know it’s really difficult to say no and that you just accept things because of your inner fear of conflict and lost opportunities. You just have to get over it and just learn once and for all how to say no! You’ll do yourself a favor… and everybody else.
The next time your boss (or anybody) is trying to push down an extra task on you just say “I’m sorry but I can’t do this right now.” in a straight and non-defensive manner. In time, learning how to say no will let you focus on the tasks at hand and it will change the habits of the people you work with – they’ll be more picky about the tasks they delegate to you.
If they ask for an explanation just say the truth – that your schedule is already full and that another task will just lower the quality of your work along with actually delaying your already existing tasks.
If they don’t understand “loss of quality” and “delay”… well, it’s their problem.
Quality is a rare commodity these days and we have to fight for it, even if it may hurt some feelings or even if it pisses some people off. In the end, quality always wins.