It was one of the first methods (self-stab the heart) that I thought of and at the time I collected some information but I deleted everything on my first try x) ...
As they have said, it is not easy at all, it seems easier than it really is. It has to be a quick and deep cut and the last thing you want to pierce is what you shouldn't, which will be hard enough to cut through the muscles that cover the rib recesses. Some of the bizarre videos that I watched in my youth I have not forgotten, among them was the open heart operation of a suicide attempt with the same method, the blade of the knife was left through what would be the "skin" or membrane covering the heart, with hardly any lacerations. If you think it is the most appropriate method for you, go ahead but think about it well
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