I regularly thought that I'm dying from a heartattack for a year or so ... it was freaking horrible. I slept with the lights on so I could find my phone quickly to call 911 and shit like that. Thank god I'm over it.
Can't really give tipps, but I was thinking about death alot in that time and I told myself it's not actually that bad if it happens, my life sucks anyways etc. wrote my will and stuff, that made it a bit better. Now that I think about it I'm actually happy about my conclusions about death and life. Basically what I learned is that life is far more scary than death if you really think about it and that your life is basically meaningless anyways, but yeah whatever .... good luck