First of all, I like how the OP assumes that this is suicide.
Nixon suffered from
Phlebitis and an overdose of anti-coagulants is believed to be the proximate cause of his death. Did Nixon know he was overdosing or did he place undue trust in the hands of his doctors?
Dr. John Lungren, Nixon's personal doctor, claimed full ignorance. But the Medical Community chose to strip his medical credentials from him. The questions which may never be answered is whether this is a mistake, or deliberate. Some claim that it was Suicide for Nixon, but it seems an odd way to go.
Nixon, of course, suffered a stroke from these chemicals which he died of thirteen months later. Indeed, that's the only reason Watergate even received national attention--Nixon's first meeting with special investigator Archibald Cox was followed by the stroke. There may be a some truth to the claim that Nixon's political enemies share some responsibility.
My thoughts is that Nixon was simply stressed out.