found pictures on partner phone

With a counselor.

You need an impartial third party in this, to talk it out, and decide whether it's worth it to hang around, particularly if you have children together or own a business or home together and have the other trappings of entwined lives.

He needs to hear it from a third party that he's got to make some choices (and probably treat you better).

Go to counseling.

This just doesn't "happen."

It takes time and many connections to get to the stage of sending mutual naked pics to each other. He's got a relationship going, for sure.

He's been cheating on you for some time.

Don't get "through" this. Get out of it.