Personally, I wouldn't be honest on this occasion, until you see whether you're offered another position. It might just be better to say that you left to widen your horizons. If your "effort, hard work & commitment've" been acknowledged, then I assume you could ask for a reference which states this? When you're offered a new job, I think THEN'S the time to come clean, and ask if it affects the decision to hire you. You could say that you really wanted the new job, and therefore thought it best not to say anything about the true reason for leaving your last position. I must say though, that you'll need a bit more effort and hard work if you're to achieve the targets expected of you. Good luck.