The simple past is fine for time periods that lie entirely in the past: "I suffered for a long time (but I'm not suffering any more)". For time periods that continue to the present, use the present perfect: "I have suffered for a long time (and I'm still suffering)". "had suffered" and "had been suffering" are both possible in your sentence, but are "more complicated" tenses, and the point in the past at which the suffering stopped needs to be somehow referenced in the surrounding context.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.