I really hate to break it to you but recently research has been conducted to show that alcohol doesn't necessarily cause promiscuity - when you give people a money incentive to control themselves whilst they are drunk then they can do it - http://www.bbc.co.uk/news/magazine-15265317 - so I don't consider alcohol to be a valid excuse.
I think when people cheat, that really should be the end of the relationship. I've been cheated on before and it is horrible - but yeah I really don't think it's possible to continue a relationship once the trust is broken unless you genuinely do feel like you have forgiven her. It's easy to say 'yeah I forgive you' but if you're going to still feel bitter and angry about it then you may as well save yourself the pain. It will feel like the worst few weeks of your life afterwards if you do break up but then one day you will wake up and realise that life is actually pretty sweet and that you've been quite a strong person to get through it.