What you need help with is HIM taking responsibility for the children so that you can have a break. How can you pay attention to him if you are worn out?
He needs to be taking care of the kids after dinner. Baths, bedtime, etc. You can take a bath, stretch out with a romance novel and get you mind into looking forward to sharing a bed with your partner for something other than sleeping. He will appreciate that. Use birth control so that you don't end up with #3.
You need to tell him that with you both working, you need help. Tell him that you know that he will do fine with the kids, and don't hover.
If you can cook several meals over the weekend or put oven ready meals into the freezer, dinner during the week will be easier. Put a load of clothes in the washer every morning after you wake up, and then throw them in the dryer as soon as you walk in the door from work. When the dryer bell rings, try to drop what you're doing and fold the clothes straight away. When you finish dinner, just go put them away. If you do at least one load a day, you won't have so much to do on your days off. And if you put the one load of clothes away immediately, the clothes take just a few minutes. This worked so well for me when I was working and then I caught up on Saturday mornings. (I was always sorry if I didn't put the clothes away and let them stack up.)
Ask him to do certain jobs in the house that will help you. Vacuuming - that would help - once a week. Taking out the trash. Make sure he is watching the kids while you do a job, or ask him to do that job.
Get a babysitter to watch your kids at least every other week. Go out with him and have a blast. Push yourself to have fun no matter how tired you are.
It's important to establish that both of you have responsibility for the house and the children. That way, one person isn't absolutely worn to a frazzle, and you both can enjoy being together more.
I hope your fiance will see how much it will mean to you both for him to lend a hand.