'He's getting scared it is taking me so long to be OK, he just wants to move forward'
It is not about what he thinks - he doesn't set the rules for how long it will take you to get past this (not 'over it') you might decide you can't get past it - it's huge betrayal. Five months is nothing, I was betrayed by my (now X) H after a 30 yr marriage - it has taken me 2 years to reach a state of equilibrium.
You have every right to take as long a time as you need to decide what you want to do and to see how you feel about him.
It is not only the betrayal that he had sex with another woman, it is the deceit for 8 years - he has effectively lied to you every day of that time, the tarnishing of your memories so that you will question all your interactions since then, his disregard for your sexual health, the fact that you will never feel quite sure of him or trust him again and the constant nagging feeling that he has only told you part of the truth.
Those are huge things to get your head round - I am not saying it is irretrievable, everyone's relationship is different, and I think it is possible to salvage some relationships, but I doubt they are ever the same afterwards. So you need to think about what your 'new' relationship with him will be like and whether you want to work towards that.
Please don't let him tell you, or tell yourself that it was a difficult time, he was looking after children, his dad had died, etc etc. There is no excuse for what he did and he shouldn't be making any, He needs to own it for what it is - a selfish, thoughtless cruel act of betrayal - other men through hard times and don't get blow jobs from random women in car parks.
I wish you luck and strength in deciding what you want to do - but don't be rushed by him because he wants to feel better quicker- he's had 8 years, and he's fretting about timescales when you've only had 5 months to absorb this news.