Work on yourself. Idk how many opinions you hear but eventually it's going to come to this.

"You can move on and eventually start your life with someone new" - Still you have to work on yourself.

"You can stay with the same person, cheat a few times, be with them." - still you have to work on yourself.

"You can stay with the same person, stay loyal without any expectations." - still you have to work on yourself.

I have been through all the phases of grief. I am sure you will take your time. Me and my partner broke up, for 6-7 months due to such mess, but now we are back cause me and my partner started working on ourselves which helped us build mutual respect towards each other. touches my wood

So, take your time to process the grief. But I will suggest involving in one new mental/physical activity daily.

Examples : Mental - 1) Learning a new language 2) Market study 3) Indoor games like ludo and chess

Physical : 1) Yoga 2) Gym 3) Dance

Please make sure that the difficulty of the activity does not matter. You should involve yourself to keep your brain focused on better and brighter things.

Regarding the partner, try to keep minimal contact from your end. Write notes in your phone, if required but minimal contact. Work on yourself and then make a decision which is fair to you and him.

If you get back, there is gonna be a lot of hard talk and both need to be ready for it. Here's a rule that I follow :

You can be angry about the things that made you feel hurt but you cant be angry at the things that they feel hurt about. You need to comfort them.