An 18 month betrayal is a pretty big thing to forgive and forget so I'm not surprised you are still struggling after only 2 years. The reality is you will never forget, even if you do partially forgive, but it isn't something that goes away with time.
What time does is allow you to start again if that is what you and to do and build what is effectively a new relationship with your DH based on his behaviour and attitude towards you now, and with a new inner strength because you know within yourself that everything that happens in your life now is your choice.
These threads attract a polarity of opinion, from those who have tried to make it work and failed, those who were not given the option of making it work, those who left, those who left but subsequently reconciled. Each person brings with them some fairly inflexible views. So my advice is to remember this is your life, so do what feels right for you.
Me, I'm in the left but reconciled camp. So my view is it can work, if both parties want it to, but it will always hurt. The hurt is within though, it stays with you regardless of whether you stay in the marriage or not. If does alter your attitudes towards people, and make you less trusting. Not necessarily a bad thing. Let's just say your bstard radar becomes very finely tuned 
Good luck. It gets easier. I'm happy. My marriage is happy. There was a time when I never thought I'd say that.