After a divorce is finalized, can the settlement be renegotiated based upon new evidence?

My divorce has been finalized for over a year. Prior to our separation (the date he left the marital property), my ex got his girlfriend pregnant (I obviously did not know this). Thus, I could not file on the grounds of infidelity (I knew he was cheating, but my attorney said I needed proof of intercourse. I assume getting his girlfriend pregnant is proof enough). Because I had to file a no-fault divorce, I ended up having to pay him $18,000 as a settlement for his portion of the marital property. So, since I now know that my ex was indeed unfaithful and committed infidelity, can the settlement of $18,000 be renegotiated? It surely doesn't seem like he deserves half of the marital assets. I am paying him $275 a month for seven years or so.