Needless to say, you two have a lot of talking to do. My guess is that you will need a couples counselor to help you both express yourselves and hear what the other is saying. This issue is so serious that it is understandable that you haven’t been able to get past the hurt and guilt and anger so that you can communicate the feelings and issues that are between you.
One guess I have is that your fiance freaked out about the responsibilites of being a young father and acted out on an impulse that he later regretted. It’s possible he didn’t tell you because he just wanted to erase the whole experience. Of course, he can’t. But he isn’t the first to try to pretend that something shameful didn’t happen.
I’m concerned that we didn’t get this letter until after you had to make a decision about the wedding. Whether or not you went through with getting married, you two do need to deal with this. If you did marry, you both need to fully understand what happened so you can both get to a place of forgiveness. He needs to forgive himself as much as you need to find a way to be at peace with forgiving him.
If you didn’t marry, I hope you will still see a counselor. You two have three years and a child between you. You can’t walk away from involvement with each other because of your responsibilities as co-parents. It would be a shame to write off what could be a good marriage because of a momentary lapse of judgment, even one as serious as this one.
I think you two should give each other a chance. See a counselor. Get to the bottom of what happened. Then make your decisions. All three of you deserve to get some resolution on this.
I wish you well.