Marriage/Divorce

Hi there, 
This is my first time posting on here and I know these aren't usually the issues that come up on here but I really just need to get this out and would love some kind words from women who maybe have been through the same or know someone who has...
I'm 25 years old. I have a master's degree, and starting my first year of teaching this fall. I'll be teaching first grade. I've been married for almost 3.5 years. My husband and I, however, have been separated for almost a year. We have a two year old son. When our son was 4 months old, my husband cheated on me. I was devastated. He said he was looking for affection because I wasn't giving him any. I drove myself crazy wondering what she looked like. I never really found out, aside from the disgusting details I somehow got him to confess... I had to know, in a weird way. I felt sooo betrayed. I was very conflicted, because all my life I said I would never stay with a man if he cheated on me. But having a baby with him really altered my reasoning. I stayed, I "forgave him" but I never really did. 3 months later, after months of driving myself crazy with disgusting and hateful thoughts, and not feeling like he ever understood how hurt and betrayed I felt, I made the (wrong) choice of cheating on him back. With two guys. Yes I know, it was dumb, but at the time I needed him to feel the pain I was feeling. Eventually he found out, and was just as hurt as I was- but worse, because he got to read my conversations that I had with these guys on my computer- at the time I didn't know my laptop had synced with my iPhone and all of my messages that had been deleted from my phone could still be found on my computer. Honestly, it felt good to see him hurt like I was. He finally understood. (I know, I sound like a complete bitch. I'm not, I swear, I just wasn't in the right place emotionally... He really really hurt me) 
Fast forward a few months... I had a miscarriage. I didn't know I was pregnant, and certainly wasn't trying to get pregnant while having a 6 month old baby boy. That really messed me up even more, hormonally and emotionally. My husband at the time had made a promise that he would stop everything as long as I stopped everything. We made a promise to play no more games. But I still didn't really care. I stayed with him of course, but I never cared if my actions hurt him or not. I never slept with anyone again, but I did text people behind his back. I'm not sure if he did, because I was in grad school at the time and never really saw him until I got home late at night. 
Maybe like 6 months later, I decided I was done with the games, the drama, the abuse, the yelling... Everything. So I left him. This is were the separation started. I took our son and went to my parents house. One day, the doorbell rang and I opened up to be served with divorce papers. I wasn't surprised. We were set to go to court 3 months later. This entire time we had been apart, we hadn't spoken at all. I tried online dating, which didn't go well of course. 
It was January 2016 and the day came when we went to court, and I saw him for the first time. I was so nervous to see him again and I cried. I cried from all the emotions that came to me, I missed him, I felt bad for what I did, I missed our family, and I didn't want to be a single mom. We talked that day, and he said he didn't want a divorce. It was his only way to fight back, he said. We decided we would go to counseling, and try to work things out.
To summarize, he's still living in the past. He's very hurt, obviously, but he hasn't forgiven me. This hate, bitterness, and anger he has towards me have prevented him to actually working things out with me. Every time we had an argument, he leaves because "he doesn't have to put up with this shit"... He walks out on our son and I and doesn't even phase him. Or it seems like it. My entire gut has been telling me for a while that this isn't gonna work out. Something keeps telling me I have to leave, and there's someone better for me. But I'm ridiculously stubborn and keep coming back to him the second he sends me a loving text or shows up at my house begging for forgiveness. I've been so weak... I've forgiven him every time. I've always had a feeling that he was up to something, but when I ask him, he of course doesn't tell the truth but I still choose believe him while my gut tells me he's lying. When he's with me, he always has his phone in his pocket, on vibrate, with a passcode on it. He doesn't take his phone out ONCE when we are together and when i ask him, he says "it's because I want to focus on us". My intuition keeps telling me he's lying, but I believe him.
Just the other day, we went out to dinner and a movie. Everything was well until this day. He pulled out his phone to look something up, and as soon as he unlocked it, it pulls up his text messages screen. I see a text from a girl named "Ella". My blood starts to boil because at that moment I know my gut feeling and intuition has been right. I ask him, "who is she?" And he says, "this girl from work" I thought, ok, maybe he's being honest so I said "ok, show me the texts" and he said "I don't have to show you shit"... I told him, "if you don't show me, I'm leaving" and he says "you're outta here, ok, see ya" 
That's when I knew... This is why he doesn't pull his phone out when he's with me. This is why he walks out on us so easily. This is why he doesn't feel like he has to put up with this shit, because there's another girl waiting there for him when he leaves us ... 
I wonder if she knows the real him. I wonder if she's weak and thinks she's gonna change him. I wonder if she's amazing, more than me, and that's why he's talking to her...
Anyway that was like a wake up call for me. I told him I was going through with our divorce, because he would never be a one-woman man. I haven't talked to him since that day... Usually he would have texted me already saying that he loves me and that we can work it out... But he hasn't. It's because he's got her :( 
I'm having trouble staying strong. Even though I know I deserve better,  I'm having trouble thinking that if I do go through with this divorce, I'm going to be alone. A single mom, divorced at 25. I feel like a failure. I feel like I've failed my son for not being able to give him the family he deserves.
I need help... I know this is the longest post ever, but I just hope someone reads it and can help... Sometimes a complete stranger can have the right words. I just need a friend, something... I know this is so depressing. I just hope someone reads it. At least... 
Thank you for reading this if you're already at this point. Just know I'm thankful that you've read my story...