I am a single mother, putting herself through University and working full time.

My ex and I broke up 6 months ago, when we broke up he told me it was because he couldn't do a relationship and do University, even though we had both been doing it for 2 year. I asked him if it was because I had a child and he wasn't ready to make that commitment, and he said no, I asked him if it was because I made him unhappy, and he told me I made him happier than he thought he could ever be and that he loved me. I asked if there was someone else and he said no, I told him that if he was lying and if he moved on after telling me all this it would crush me, and he said that he wouldn't do that to me, that he couldn't imagine being with anyone else he just needed to be alone and focus on his school. Then 2 months after he came and saw me and told me how much he loved me again and again.

Then 2 months ago I got a text that said he was seeing someone seriously, when I asked him why he lied, he said it was only working with her because she lived in another city, then I found out that he is moving her to our town, and she will be going to University here. And I found out they were talking when we were together, and when he came and saw me he had already been with her. I found out she is 18, and he has never been happier.

I want to text/email him and tell him how much he hurt me, and how honesty would have been so much easier. But I am afraid to come across as "that crazy ex", which he has been telling people I was his "crazy girlfriend" and that is why we broke up. But he has never once given me a straight answer as to why he really broke up with me, and I really want him to say he is sorry and that he should have been honest. Now I have to see him every week in class and one of my so called friends lives with him. I don't know what to do anymore, I am trying a new relationship but I am not over my ex (the guy I am seeing knows this and is prepared to wait, which makes me feel bad, I can't be a good girlfriend). Am I being stupid/crazy? Should I contact him? Would it make things worse? He promised me so much and kept telling me he loved me and strung me along, and I want to tell him that it wasn't fair I deserved more. Should I try and just let it go and be happy for him?

I think you should confront him alone and tell him exactly how you feel. Tell him that what he did you didn't deserve it. After that forget all about this guy, let him go because you really do deserve better than him. Also this new guy you mentioned, try hanging out with him if your interested in him or you can go out with friends. When you see you ex in class ignore him. Don't let him see you hurt over the situation but let him see you strong and that you not going to lose any sleep over him. Always smile and try your best to do what's right for you and your child, BE HAPPY.

Instead of putting yourself through the pain and distraction of holding on to this man (or worse, considering yourself not "good enough" and tearing your own self esteem down) - let go. You don't have a choice. In every way he can he's said to you "I don't want to be in a committed relationship with you" and he's moved on to someone else. Why doesn't matter. It certainly doesn't mean you're not good enough nor that you can't have another guy love and adore you and want to be with you.

You do NOT need to know "why" because it doesn't have anything to do with how good you are or how worthy you are. Nothing THIS GUY can tell you is going to give that key to your heart or help you let go. You have to do that for yourself. All this means is that you and he didn't connect completely and that he didn't feel a permanent emotional and physical attraction to you. (and him saying you're the "crazy ex" is a signal that he is not worth your heartache)

Move on with your life and look towards those who are ready to come in to your life and form that special connection with you that you hoped you could have with this ex.

You are the ex, there is no reason to do anything but forget and get over him. Doing anything right now has no meaning at all. He lied because he did not want to get into a fight over this, and doing anything now will not help any at all.

1. stop texting him, stop emailing him,
2. don't open or read anything he sends.
3. don't answer the phone if he calls.

Just go on and move on. You have not done that yet or you would not care what he did or does.

But we never fought about anything, and why wouldn't he just tell me the truth? And why come and see me and tell me how much he loved and missed me just to turn around and start a new relationship days later?

People withhold all manner of their true feelings. Often one partner thinks things are just fine ("we never fought" "things were great") because their partner doesn't have the guts to be direct and are protecting their own feelings (avoiding conflict) above being honest.
And why are they afraid of being honest? Because they don't want the emotional responsibility nor are they equip with dealing with the fall-out and they don't want to deal with a distraught or needy partner so they "act" until they see that first opening to flee or the truth comes barreling out. :(

Sorry to say this but:
You have to come to peace with what "IS" not ask WHY it is because that doesn't change what is happening now and what you ultimately have to deal with.

(the "Truth" in this case being that he met another girl and was interested in her)