I'm in a bit of a 'love' pickle...?

So basically I split up with my ex-boyfriend just over a month a go now. We had very good reasons for the break up, in that he was a complete idiot to me & emotionally controlled me for a large proportion of the relationship. Realistically, it was only relatively nice for the first 6 months and it took me over a year and a half to split up with him because I was still very under his influence and much in love. But anyway, there's this girl who I've always had to put up with as being a bit of a b*tch sometimes, but one could say that's just her personality, so everybodys used to it. But since I've broken up with my boyfriend, she's made a concious effort to make absoloutely everyones life misery. She's been spoken to on several occasions about it and how she has to stop it, and especially me, because I genuinly haven't done anything wrong and she knows this. I then found out from one of my close friends last night, that she's actually been in love with my ex for over a year now, and despite my friends' concious efforts of telling her that she shouldn't surely because of how badly he treated me, but she's still convinced that he can do better for her. The thing is, is he's playing her off of two other girls at the same time. My other friend is quite close to him as well and rumour has it she likes him too (God knows why, he's a d*ck). But he's either pretending, or leading them on by saying he's fancied ANOTHER girl for over 6 months now. And yes, that would make 5 months of our relationship time also. For one, is this even possible? To be apparently so smitten in love with someone as he lied to me and then ask around if it's okay for him to also fancy someone else at the same time? He made me feel so bad when I broke up with him that I actually punished myself on several occasions, and upon hearing this I'm so angry that he's still messing around with peoples heads, those being three of my once very close friends. Now the first one whose in love with him is aparently raging with jealousy because she saw that I could have him, and now aparently he doesn't want her, yet is still leading her along majorly. He's also being silly and has completely switched friendship groups TO the girl that he says he's fancied for 6 months, and the other girl is stuck between all of these dialemmas and has to put up with constant bitching from other people, like I do. This means that the first girl has followed him in the hope that if he's with him whilst he's with a girl he fancies, she can try and win him over. He's been going out with them on 'dates' if you like as well, taking each to the cinema and going out to places with them, outrageously flirting via text. This is a complete mess, and I'm not even supposed to know any of this so I don't know what I can do to help, but when I did find out about everyones feelings, I was no longer angry at this girl, but at my ex boyfriend for even thinking he can try and convince someone he's any good for them. I don't want any of my friends to get hurt, but they're already hurting and I'm scared it'll get worse. I'm not even over him yet, and keep thinking back to the 'good' days over a year a go in the hope they'll come back. What do I do? I have this overwhelming urge to slap him and humiliate him in front of everyone, just like he humiliated and hurt me for 2 years, (I won't - but it's a very strong urge I'm resisting) and I just want this to all go away. I keep on going through phases of absoloutely hating his guts for ever, EVER thinking he could treat me the way he did, yet when my friends tell me this & say I did the right thing (and this is well over 15 seperate people who've said it) I go back to thinking, but what if he actually did love me and I've just ruined a potential for the future? I'm still in love with him, and he's just being a complete player and ruining my chances of staying friends with this girl, whilst leading 2 others on. I can't even believe he thinks he's over me enough to go out on dates with other people after less than a month...