I don't know where it comes from but I get it a lot, and I sometimes say it out loud without really noticing, I feel like I'm cursing myself too and that I'll end up getting cancer or something because I'm wishing death on myself. I'm not really wishing death on myself though, because I don't want to die, the thought of wanting to kill myself just appears out of nowhere, I'm not really that good at explaining things, sorry.