If you are working so hard that you are stressed to the breaking point, see your physician or psychologist, with the possibility of receiving medical clearance to go on sick leave /stress leave. If you can’t sleep because you’re working so hard, or work is desrupting your otherwise normal life, seriously think about this option. Your medical plan covers this and if you don’t have insurance, then you can approach this from a workplace health & safety perspective / unemployment insurance. A hunk of every one of your paychecks goes to fund these goverment programs, so USE THEM. There is nothing wrong with this, and it doesn’t mean you’re ‘weak’.
Recognize that your employer needs you more than you need them. I say this because if you are filling two roles for the price of one, your company probably cannot do without you. While this is tougher for people to do when they have less experience (aren’t wise to ‘normal’ employee / employer expectations), know that you can establish boundaries and expectations. Advise your manager that you will simply not be able to fulfil the duties of BOTH positions and they’re going to have to pick the ONE that you will be responsible for. If they won’t play ball, tell them you intend to resign. They will do anything to keep a person that does as much work as you, including cutting your hours substantially.
Realize that salary does not mean ‘you will complete as much work as we tell you to, even if it means sleeping under your desk’. There are labor laws which govern this type of employee abuse. Spend a few hours calling local government agencies and find out what your rights are. Better yet, consult a labor lawyer. He may actually be able to extract money for all the extra hours you put in. Plus, the company basically can’t fire you after this, because that getting fired for attempting to protect yourself from abusive employment practices is called ‘wrongful dismissal’, and is worth even more money to you in a lawsuit.
All in all, finally I can just tell you to look out for yourself first. Your company isn’t looking out for you, so YOU HAVE TO DO IT. You may have a strong work ethic, and want to do good for the company and it’s clients, but at the end of the day, are you reaping the profits? NO. So always look out for #1.