Have you been at the job long? Have you had any problems or write ups at work? I would think if you've been there awhile and aren't a problem then they would respect you for your choice of wanting to function at optimal level before returning. And if I were you I'd put something in there like that. That you don't feel like you are up to par yet and if they are paying you, you want to do well for them. (turning it to make it be about the company is always a good idea because to employers, it is.) I'd also let them know (if you know) a date when you will be returning as well as a plan to what you will be doing during that time off to improve bring yourself to where you are able to work again.
Just my two cents. I've considered asking for FMLA and short term disabilty before to go away and get some treatment, but so far I haven't. Partly because I'm scared to do so and partly because I really can't afford it. So take it with a grain of salt.