Military Divorce: Can a divorce settlement be amended almost four years later when it is found to be incorrectly calculated?

I got divorced almost four years ago and found out recently that I got a bad deal on my settlement in regards to my military retirement/pension. I was told by a lawyer who performed as our mediator during the divorce process that my Ex was entitled up to 50% of my military pension. This was due to a 10/10 rule that entitled that. She of course ran with that and will now receive half of my retirement due to how the settlement was written. I have since learned that 50% should have been based on the amount of time married while I was active or something along those lines.. Can I file to amend the settlement and have this correctly calculated? Details: I plan to do 20 years total. 12 of the 20 will have been married while active.