P.M.
Sweetheart, the changes to your dreams and expectations have come as a huge shock to you. This is recent, and you are still reeling. Take things one day, one breath, at a time. You will gradually begin to see a way forward. None of us out here can tell you exactly what that might be, but we can tell you some things about life and love that are true.
Here's my list of suggestions, mostly lessons from my own life:
It's possible your husband married you in a sincere attempt to pull himself up to the standard of worthiness. You can't know that he ever really intended to deceive you or abandon you. He may have been fighting against that. At any rate, it will do you no good to wonder about his intentions. You really can only work with your own. But it will be worth the effort, for your own sake and for your children's, to intentionally work toward forgiveness.
When your son cries for his daddy, hold him and empathize. "Oh, yes, I KNOW how much you want your daddy. You wish your daddy were here. Me, too. But Daddy can't be here right now. So let's cuddle together until we feel better, okay?"
Your son is young enough that his feelings will probably start to shift fairly quickly, if you don't lay your own emotional burden on him. Remember that you want him to be emotionally healthy, and for that, you will need to allow whatever emotional growth this situation can bring. And EVERY event, no matter how tragic, can bring emotional growth if that is our intention.
Keep your mind off your husband by keeping it on whatever is good, positive, and constructive in your life. If there's not much of that, then make more. Eat thoughtfully, exercise, take walks, read good books, take classes, join a club or organization, become part of a mom's group, MAKE opportunities for yourself. If you don't have a religious practice, find one that fits your personality and lifestyle. Avoid drugs and alcohol – they are devious "friends" when one is in pain.
You won't stop thinking of him completely. That's just the way it is. But you'll think of him less often, and with less pain, as you practice. And time away from him and WITH YOURSELF can be very healing. You're young, and have had very little time to learn who you are as an individual, rather than as someone's daughter, girlfriend, wife. That's part of what is making this sudden loss feel so impossibly hard.
Don't plan to start dating right away. That's just one more devious way to try to run away from pain. It may seem to work, but you will be much too vulnerable, and could develop unhealthy dependencies on romance, sex, or the wrong guy(s). After you are divorced, and when you get to know yourself better and have a clear sense of what you do and don't want from your life, then it's okay to start dating. But keep your date life and family life separate, until you find someone really worth your kids thinking about as a stepdad. Be really sure about a commitment before your kids form a bond. My mom brought a dashing potential "dad" into my life, and he left when I was eight. It was the single most devastating event I ever had to endure – and has marked every relationship I've had since. Don't do that to your children.
On hope: it's worth quite a lot in helping us move forward. It's also potentially cruel. There is some chance that your husband, being young, will sow some wild oats and then discover it's really you he wants. You may have a chance to take him back, if you want. But don't live on that hope, or it may blind you to better alternatives.
Give this all some time. You can and will get up and continue living every day. At first it might be only for your precious boy, your coming baby. That's enough for starters. Eventually you'll notice that you are also living for you, for new dreams, for the excitement of what you're learning. Go for it.