"i am surprised that even atheists consider themselves sinless"

I consider myself sinless. I've done stupid, bad and thoughtless things, hurt others and made bad choices. I've hopefully learned from these experiences. But I've never "sinned", because that's a purely religious concept and I'm not religious.

As to this issue about mental ill health being rooted in guilt, maybe it is in some instances. What the average mental health practitioner would say, though, is that more often or not it's rooted in trauma.