Summarize:

When you talk to your boss it is important that you do not flat out say "no" if you can possibly avoid it. Rather try to find a way of offering a positive alternative that still amounts to you not doing the task you feel you are unable to complete. One good way to do this is to suggest your boss helps you reorganise your priorities. By doing this you'll be showing your boss that you are seeking to be as productive as possible, but you will also make it clear that your workload as it stands is not sustainable.  Take a record of your outstanding work and how long it will take you complete each task to evidence that you have thought about it closely. Asking your boss "can you help me reorganise my priorities?" will demonstrate that you want to involve them in how you manage your work. This will show you respect her opinion and are seeking some guidance about how to work more efficiently. Another way to suggest a positive alternative rather than just saying no, is to recommend a colleague who could pick up the extra work. Doing this will demonstrate that you have thought about the task and who is best suited to it. Your boss will be impressed that you have thought about her and the company's need to get the job done rather than just your own concerns about being overworked.  Demonstrating that you have good judgement and have prioritized the needs to company will make it more likely that she trusts your judgement more readily in the future.  You will also demonstrate that you understand what is happening throughout the office and are interested in the development of your colleagues. If you are being given too much work to complete in the hours you are contracted to work, this could be an opportunity to approach your boss about establishing a new working arrangement. For example, if you have a long commute that is eating into your productivity you might suggest working from home one day a week which will cut down on your commuting time.  If you think a more flexible working pattern would enable you to adapt to different demands at work, don't be afraid to bring this up. Always think about the culture of your workplace and whether more flexible working is a feasible idea. Clearly think through any proposal before you make it. Don't go in with an idea that you have not fully fleshed out.
Suggest reorganizing your priorities. Recommend a colleague. Propose new work arrangements.