I did SH a bit between 18 and 20, due to depression. The urges basically went away after that, only to resurface about 6 months ago, which was about 18 years later. In my case, they just appeared in a couple of major depressive episodes (though they've been lingering a bit lately). I think the issue is that once your brain sees that as a sort of release (or self-punishment or both), it's easier to go there again. My T has been talking about finding alternatives if I'm in an emotional place where those thoughts are likely to come back. Like do something such as talking to someone, writing up what I'm feeling, etc., in an attempt to keep myself from feeling the urges.