Online Co-op made me skip a mission

To try the online co-op, I played an matchmaking campaign game. Mission was Neon Arterial, I got transported to the vault just before the boss battle.
I don’t mind the spoiler, but now I don’t have the tool for breaking eridium, how do I get a hold of that? is there anyway for me to roll back my save game or reset to the previous mission and do it again.

Thank you.

Beneath the Meridian! is the mission I need to get back, the entire mission was skipped.

edit

I went back in to fight the boss and while the object didn’t appear for me to take, my melee changed so I could break the eridium. At least I don’t feel the need to start from scratch. But if there was a secondary save or an option to go back to an earlier part of the playthrough, it would help in alleviating stress and annoyance.