Job-hunting often consumes a lot of time and energy and if you´re working 40h a week or more, it can be difficult to devote yourself to it.
The most important thing is to NOT do your job-seeking at work. Even if it´s going to be difficult, you have to keep your job separate from your job research. It´s unethical and If your boss finds out, he or she may take it personally and see it as a lack of loyalty. You could not work there sooner that you thought. Moreover, if you focus too much on your job-hunting, you may lose your attention on your current job. Remember that your researches are personal and as any personal issues, they shouldn´t step in your daily work. You must keep giving your current position the attention and respect required.
When you´re at the interview, remember that you’re much more attractive than other candidates that are not currently working. Having a job shows you´re reliable and you´re wanted. Nevertheless, the employer may ask you why you want to quit. Bad mouthing your boss won´t help you, and will be perceived as a huge lake of ethic, loyalty and professionalism. If you quit the job because you can´t stand the boss anymore, you should better say that you want to explore all the opportunities you have, and change company to learn and develop new skills.
You also can ask the interviewer some discretion and to not call your boss for references. Employers can understand your situation.
Although you think you can´t handle this rhythm anymore, you shouldn´t quit your job before having a new one. It could be very risky. Instead, take breaks, go to take something outside, take a walk, meet some friends or do some sports. Find an activity that keep you away from the job subject. Change your mind and remember to not pressure you too much. You still have a job, so take a deep breath and don´t be in a rush, each thing on its time!
Finally, remember that when you find your new job, you have to leave your old one with your reputation intact. Finish strong and leave on a positive note!