Eh, I get that quite often too. Comes with mixed in paranoia sorta. Not to mention the all-the-time lack of self-esteem and feeling generally failed in life due lack of any meaningful relationships and un-called for self critique..
Considering first time I wanted to leave this mortal state behind was about 15 years ago I can only say I've had some time to think about it. Hell, back in mandatory school I had detailed plans on how to end my miserable existance in a way to give my bulliers' life-long traumas. That was before I studied psychology for 3 years, which made the plans quite a bit more polished..
Around then I just realized that if I'd do it, I'd leave behind devasted parents who've spend countless hours and money to push me towards doing something meaningful with my life. So I've postponed my final journey until everyone I care about has left before me.
Considering how long my mother is likely to live that leaves me with decade or couple to find something worth staying around for after she leaves. *Shrug.* So when anyone concerned of my mental health asks me about suicidical thoughts I can honestly say I haven't had any of those for a long while. It's just well thought-out and planned option still.