UPDATE: How do I (M40) tell my kids (F12/15/17) that my infidelity is the reason for the divorce?

I took everyone's advice and involved my ex-wife in this situation. Safe to say she was pissed off at my poor choice in partners and having to deal with this bs. But we got together with the girls at ex-wife's house and told them.

Many of you suspected the girls already knew. Incorrect. Apparently they thought we broke up because of my job and my refusal to accept a new one. We did not lie to them during the divorce and tell them an untrue reason. We just told them that relationship issues are private and we would be willing to have a discussion when they were all adults. So thankfully there is no trust lost in my ex-wife which some of you were concerned about.

All my girls were incredibly upset. They were being cordial with my gf because their mom encouraged them to be civil but apparently hated her. The couldn't believe I would give up my wife for her, our family and their stability. Which is fair. They are refusing to see me for now, my ex and I are going to get them into therapy and I'm hopeful that over time we can rebuild a relationship. My oldest is furious and has sent me some scathing messages and then blocked me, so I am really concerned about her and I. I missed some of her huge life events due to my affair and I think that will be hot for her to forgive.

As for the gf, she is gone plain and simple. She has her next partner lined up and is moving to be with him. I can't say I'm hurt about that, which shows me how little I cared for her, I just feel relieved.

A bunch of you suggested getting back with my ex-wife. Unfortunately, that topic has been breached when the affair was discovered and she was/is 100% uninterested. She takes her job as a mom very seriously and in her mind, the girls would eventually know and it would be setting a bad example for them to take back a cheater. Even independently of that, cheating has always been a deal breaker for her.

I feel bad as she was planning on moving her partner in and now feels like she has to delay it until the girls are in a better place. She was really angry with me especially for that reason. In her anger she's also let it slip that her and her partner are planning on marrying in the next few years and having another child. She's never shared anything personal with me past allowing me to meet her partner before the girls did so that was a shock to me. I'm trying to process that information. That hurts more than my gf cheating and leaving which tells me I have a lot more work to do in getting over her. There is a permanence in her marrying that I am not prepared for.

It's been a sobering experience for me. Thank you to everyone who legitimately gave me advice, it was very helpful. If I could in turn offer some advice, don't cheat on your partner. The grass is greener where you water it and if I had taken the time/effort/energy I gave my affair partner and invested it in myself/my wife, I would be incredibly happy right now.

And as a side note to all the men messaging/commenting/congratulating me for getting a younger woman, that shit doesn't matter. I've never looked at my ex wife and thought she was old or unattractive. And she was better in bed than my ex-gf. This had nothing to do with getting a younger woman/"trading" my wife in, it had to do with a self destructive ego that has led to the most important people in my life hating me. All that shit is irrelevant, don't get caught up in it.