Ezreal's lategame Muramana issues

So I had played a game as Ez a while back, and ended up 12/3, which gave me a pretty big lead on the enemy team. However, after I got the manamune upgrade I couldn't spam my Q for very long without me running out of mana, mostly towards the end of the game around 45 mins long. That's why we lost the Baron contest and eventually the game after - I simply got OOM. I am wondering what I should do in these situations since the enemy just waits till we back, around every 2 mins due to the mana issues, and then groups. Usually this never happens in a stomp but the enemy had a couple tanks who just healed all the damage I did to them... Ez scales pretty hard late but the new manamune just wastes mana like mad...