Wow. This sounds almost exactly like a situation one of my friends is in right now! He was married to a girl (also a friend of mine) for around 9 years too & just found out she was talking to her ex, which has happened before and they split up too. Wow. How strange that your situation is so similar.
Anyways, sorry about that & I am very sorry to hear about your situation. I really am sorry. I could not even imagine how devastating it would be after being together for so long. My partner and I was out trying to cheer up our friend until 11:00 tonight & goodness! I can imagine it's very painful for you too to be dealing with this.
My first thought while reading your story was when I came to the part about you getting with the new person who treated you amazingly -- I thought "good for you!" That's good, but then I seen you said you took your ex back because he begged.
I don't know if you can really fall in love with someone that fast. I mean, you said you and your new guy had already told each other "I love you" within a few weeks, so if you were able to, then I'm sure your ex is able as well. But that doesn't mean it's love. I can understand you missing him. That's natural being that you've been together for 9 years. It'd definitely be very hard to go on without someone who you've been used to being with for that long, so I can see how you two would keep getting back together & have a hard time staying apart, even if you do find others you have strong feelings for. To me, I think it's probably so natural to be with that person that you keep missing them and feel drawn to get back together. Also, it's probably comfortable, it's what you've known for 9 years. & possibly even you may both get nervous or something when being in new relationships and end up running back or falling back on each other because that's what feels right since you've been together so long.
However, at this point, it sounds to me it's time to take it to the next level and work on being apart. Unless you both think you can somehow manage to work through such a big thing -- being with other people, betrayal, etc. -- and truly love each other, it's only going to prolong the painful process of splitting up.
These are just feelings I think I'd have personally if my partner and I were to go through this, so I don't know & I don't know your relationship besides what you've shared, so I can only say what I think from what you've said, as an outsider over here. IMO, it just doesn't seem very likely to work out given everything that has happened. It seems more like it's only going make things more painful and tough on the both of you, possibly hurt each other worse & then be back to splitting up for good again if you continue to keep each other close in your lives. But again, I'm not saying you won't ever be able to make it with that person. I don't know that & it's definitely possible, but the likelihood of everything being able smoothly to go back to a normal, healthy relationship probably isn't possible.
I'd be asking myself questions like "do I really love and want to go back to this person and really deal with everything or would it just be the comfortable, easy option?" I'd also be thinking real hard about whether or not I want to put myself on the line -- deal with betrayal and be willing to overcome and totally forgive and move on. Personally, I would not be able to get over betrayal and move on with a person. Once it happens, it wouldn't leave my mind and I think it would wreck my emotions to go back and try to let go and accept rather than accept the pain of being hurt, having to split and start over. I think the latter would be easiest for me, personally to deal with. I'm all for chances, but goodness. What's that... screw me once, shame on you, screw me twice, shame on me? I think one time would do me in & two times and I'd be like forget it. I wouldn't be able to give myself (my heart and all) away again after a second time. I'd have to think about my own self & for me, that would include getting rid of the person who did that.
It was the same girl both times, right? I just don't know about it. If you could do it once, okay maybe that can be worked out, but twice? It'd make me feel like there's definitely something more there & that they definitely made a perfectly sane & conscious choice to take up some kind of feelings/relationship with someone besides me. In my eyes, it's either me and only me, or no me at all.
After finding someone who you felt you loved or could love & had strong feelings for & they returned those feelings and treated you amazingly, as you said, I think I'd be choosing the new option -- to take a chance on something that seems good rather than take a chance on being screwed over for a third time. You may not always have that good option, he may not want to get back together if you've left him for the ex & I can say one thing -- good ones only come a long ever so often. I personally snagged my good option and held onto him for dear life when he came along. I was so ready to be done with being treated horribly & even though in the beginning I thought there must be something bad since it all seemed so wonderful. He treated me better than great, he was just truly a wonderful man. Exactly what I'd always thought of as the "perfect" one for me. I couldn't take the chance on staying with the crappy guys who I kept taking back anymore. I had to choose the option that seemed good, even though I thought it sometime soon, something would have to happen because I wasn't used to such good guys in my life.
7 years later & I'm still with him, so I can't help but want to say go back to the guy you met who you said treated you well, but I don't know for sure what's right for you. I guess you'll only be able to do what your heart is wanting the most & it kind of sounds like right now, your heart might not be done with the ex. So if it so happens you can't help but give it another shot, just remember to guard yourself in the meantime, be prepared.
Also, I think it'd be helpful to consider how your relationship has been overtime, not only when these bad things happened. Were you guys always good with each other (besides the normal stuff that happens between a couple), were you happy, truly? Did you both feel like you really appreciated and cared deeply for each other? Or were things getting rocky, lots of unneccessary fighting or bickering? Hatefulness or rudeness to each other, or any signs of things going wrong, besides of course the other people.
Another thing, try to really think hard about the future. Does it seem likey you two could last and be happy and be able to really get over the bad stuff and not harbor bad feelings? Does it seem possible you guys could both manage together without getting feelings for other people? Sometimes if you think honestly and hard enough, you can see the possibilities of how things are likely to work out. Intuition or just guidance of your heart maybe.
PS. I highly doubt he hates you. A lot of times, guys will be hateful and claim to hate you because they're still bitter about being hurt, but they actually really, really care.
Will he ever miss you? 9 years... absolutely he will miss you. I'd bet he misses you right now, but would not admit it.