There can be forgiveness but there responsibility for repairing the damage needs to rest with the person who did the damage. The other issues can worked out but without trust the rest doesn't matter. As long as she is giving excuses for why she cheated and laying the blame on you nothing will change.

I know someone people can make open relationships work but I suspect the healthy ones are much less than someone people would have us believe. For something like that to have any chance of working both parties need to embrace, if one is just accepting it as condition to continue the relationship I think it could lead to some really dark places.

Of course all that being said I should mention I suck at relationships so most of my advice comes from a base of bad decisions.