I find that saying "It's all water under the bridge now" is not usually accurate. Because really, whatever it was, it was probably a bigger deal than that, and you are just lying to yourself if you say otherwise, right? So try this improved saying:
"It's all bridge under the water, now."
Which is to say, it's all fucked, and there's nothing we can do. For example:
"Yeah, we used to be super close, until he fucked me over, but that's all bridge under the water, now."
Let me know how this improves YOUR life!