It has been very difficult because I work part time and homeschool and am the one taking care of the 3 kids while his out doing who knows what. Then he gets to pop in and be the "fun parent" and play with them.
Sorry to hear about your situation, and the likely affair. A couple things crossed my mind. I know they're not your key questions at this point--just food for though.
First, if being the 100% parent is new to you, do finances allow you to hire help so you can take breaks? This could be childcare so you can go out solo--but it could also be a cleaning service or a laundry service. Anything that allows you a break!
Second, if he's leaving you with most of the childcare responsibilities, he would be expected to pay big in a divorce. If he's not paying for the home or some other reasonable amount now, I'd consider filing for temporary support payments before long.
May'19 - Girlfriend of 2yrs left. Aug'19 - We became a "broken" couple (I needed validation, she screamed). Dec'19 - We gave up and went NC. Mar'20 - I've changed and she's eager for MC and us again.