Applying for a security clearance and concerned I might fail due to mental illness. What do I tell future employers if I fail and they let me go?

I was arrested (misdemeanor) and involuntarily hospitalized a few years ago but have since been put on the right meds and have been stable for 2.5 years.

I'm not too concerned about failing since I can just get another job for a few years before reapplying but what do I tell future employers about why I was let go? The stigma is very real and I'm willing to bet a good proportion of employers won't risk taking a chance on someone whose mental illness is so bad that they will see it as a risk to their company.

Or is mental health an acceptable reason for not being granted a security clearance since it it 100% outside of my control? Thanks.

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