You say 20 years. Well, that is the problem: you should do your best to avoid letting these 20 years pass like hell on earth. You say you don't know the reason, albeit intellectually. But is it possible, and is it really important after all?
You say you made years of therapy, so I won't be naive, but if it didn't help you, maybe you didn't focus enough on what is really important: what to do NOW. Or maybe you should just try again for that bit of luck that could make it work.
You see, I don't cry out a DON'T DO IT! but I invite you to be realistic. To really committ suicide you should ahave endured WAY greater pain than now, do you really want to? You should fight hardly and relentlessly to prevent this.

Really, the best of luck