Thanks for the additional info.
I assume you meant Dec 12, 2012
, because Dec 12, 2013 hasn't occurred yet.
Anyway, from a pure jurisdictional standpoint, you can set aside an agreed-to family law
judgment within one year of date of entry, so you still have time -- but, not much. Family Code 2122(e). Grounds are fraud or mistake.
Your complaints sound as mistake or fraud, but you cannot use the mediator's testimony to try to prove that there was some sort of bias in play. The agreement itself is the real evidence. It's either fair or it's not -- and if it's not, then the court can set it aside, assuming you can explain why you were mistaken or why the other party misrepresented their position in a manner that deceived you into agreeing.
Beyond all of this, if your attorney at the time did not competently represent you, you may actually have a better claim against the attorney for malpractice, than against your ex to have the judgment set aside. So, if your new attorney thinks that the agreement is absurdly lopsided, then you need to shop the case to a few malpractice attorneys (which would be a contingency case, so you wouldn't be paying anything to try to sue your former legal counsel).
As for your chances of winning, courts strongly disfavor setting aside an agreed to judgment -- so, you must have good evidence. The agreement must really look unreasonable, such that any competent attorney would have been unlikely to have agreed to the deal. Otherwise you will likely be wasting your time -- and throwing good money after bad.
Please let me know if my answer is helpful, or if I can provide further clarification or assistance.
And, thanks for using justanswer.com!