And both of you have to be willing to put in the effort. That being said, if you're in a bad spot primarily because of the actions of one partner (infidelity, addiction, neglect, etc.), that partner should also put in the lion's share of the repair work. The wronged party needs to feel like they're being won back or it can be really hard to believe that those destructive patterns won't repeat themselves.
In my situation, my husband and I made some really clear guidelines about behaviours that were and were not acceptable, and because I saw him stick to these for an extended period of time, I was able to trust that we would be able to get back to a good place.