Gently, you're less than three weeks from finding out your husband had an EA/PA. You have to go through this. There aren't any shortcuts. It takes 2-5 YEARS to heal from infidelity and rebuild a marriage.
Be patient with yourself. Obsessing is processing, and you have to process what happened.
I was still in shock at three weeks out. Heck, I will still in shock three months out.
IMO, it's too early to realistically place limits on yourself but... You might limit the time you think about it to a certain time of day. I wasn't able to do that for a couple of years for the most part but others are able to do so earlier, I think.
Really, let this happen. It hurts terribly and it will continue to hurt terribly for a long time but it will get better. What may help the most is processing it WITH your husband. If he is truly remorseful, he will be willing to help you with this.