Hurtz, I agree to wait. I think what you are feeling is similar to hysterical bonding, you are wanting to "claim" him as yours again.... It's more a biological response that a rational decision.
My marital history is similar to yours, only I'm much older now.... So I feel this is the first post I may have something to contribute--
H and I met in 9th grade. Married 3 days after I turned 18. Had DS1 at 19, DS2 at 21. I too had two young children when I was 23. We decided to,stop then. I had a surprise pregnancy and had a daughter 10 years later, when I was 30.
First: being a parent at 30 is sooooo much easier and more rewarding. We struggled in those early days. I couldn't go back to school, I tried, just too hard. I finally did in my late 20's. but until then I was dependent on him.
Second: my H affair happened after we had been married 23 years (we are almost at 26). I was one of those that said it would be a deal breaker, yet when it happened, I stayed. However, looking back, if it had happened during our early days, I would have left. After time goes by, with memories and history, it's harder to leave, it's harder to give up.
I think my point is, you have PLENTY of childbearing years left, you have plenty of time to work on your marriage, make sure it is stable, and then decide to have another child. In the meantime, as your daughters get older, go to school, get a career or some work skills. Even if you are a SAHM, have a back up plan. You already know from experience that things happen to disrupt your world. Plan for it, make sure you have a life for you, if needed.
The age difference in the children isn't bad, I know many families prefer their children to be close in age. My daughter is 9 & 11 years younger than her brothers, and they are very close. They come and take her places, they have been a huge support for her during our issues. Don't discount the power of sibling bonding and support because of age difference.
Work on your marriage for now. Work on you. A third child makes you as a couple "outnumbered" and creates it's own stress, lol. There's always time for another.....
Good luck, I hope with all my heart that you two make it. "High school sweetheart" stories are best told by couples who are still together well into retirement age....