Best thing to do is to apply for jobs while you're still in a job. It's not a good possition to be in when work is causing so much anxiety that you can't get things done or is affecting your normal functioning ability. You can only do so much and they're putting more on you, which isn't fair.

Don't stay in a job out of guilt if it's affecting your mental health, as in the long run they're not the ones who are going to suffer it's you who will. It's better to get out and into a new job while there's still a chance to turn things round than working to a point where it makes you so ill you can't work.

With the jobs that you see that you want to do even if they have sometimes things that seem unreasonable requirements. Apply for them anyway, they might just have that down as a preferance that they'd like that.