Being honest then DoIWannaKnow you are beating yourself up over not that much really in terms of the physical act.
I think most people would call what you've just described a snog and not a fumble or much more.
I can see why your DP would be willing to forgive and move on with that. Getting into how long etc would be pointless
There is still the public nature of it and the fact you would leave him for similar but you clearly have other issues in your relationship which you are entitled to keep to yourself if you want.
Being emotionally distanced, there being other things at play etc do need to be addressed but do not allow this incident to become something you allow to be the straw that broke the camel's back.
It is like you want there to be a bigger more devastating reaction from him or for him to get revenge so you can end it.
That is not fair on him as, despite what may have happened in the past, you are in the wrong this time.
He wants things to work, if you do too you should be honest with yourself and see that it is not a big deal if he does not pack a bag and throw you out on the street over a snog - even a public one with a dickhead.
Do not transfer other issues onto his reaction to this.
Give him time to clear his head about this, invest in getting closer emotionally etc to him (yes, you should be making the running for now) but work on whatever you think may have made this possible without mentioning this incident as being the reason you need to discuss whatever issue.
This incident is the symptom, work on the issues to fix your relationship long term despite it not because of it.
I think you will get a better reaction from him if it is about any issues because they are there and you want the relationship to be stronger; not because you had a low level, minor cheating incident. That's too much like locking the stable after the horse has bolted whereas wanting to get closer and stronger anyway has a long term value - even if, for now, it may seem one sided for you to be doing most of the running (just don't allow it to become you doing the running all the time or him being a doormat or martyr)
Good luck