That's a familiar feeling for me. I felt that way every time I ever took a day off work because of depression or anxiety. Then I had to take a month off while I received ECT, and my coworkers not just in my department but in the entire organization pooled together sick leave to cover me so I wouldn't suffer a financial loss. After I returned, it quickly became clear that I would no longer be able to work because of the memory loss I experienced, and I had to leave the organization altogether. I felt horribly guilty and ashamed, almost as if I'd stolen something from these very generous people.

As pegasus mentioned, your illness is no different than cancer or heart disease or injuries from an accident. I don't think you would never think less of someone because she took time away from work to undergo chemotherapy or learn to speak again after a stroke. Be as kind to yourself as you would be to any other person struggling with a serious illness, and remember that instead of letting your colleagues down, you are ensuring that they will get back a healthier, happier coworker.