Need some advice :(

[Inivisiblewar]

I chose the name Invisible War because emotions are a Invisible War that everyone fights. (I even typo'd it lol)

Ok i'm 22 years old and I will start from the beginning.

My girlfriend of 1 year and 6 months have broken up for the third time.
She has had a rough childhood and was "mistreated" at a young age by people close to her. She now has intimacy problems which she is now dealing with and seeing a counciler about. She is 18.

Let me start off by saying I love this girl so much. And she loves me.

Just a bit before our first breakup, she started to like this guy from work because we were going through a rough patch and he was there for her. She tells me she likes him at a party and I get very upset as I am very in love with her and we are still together. We sort it out and it turned out to be nothing, just a little school girl crush. On our first break up she goes with on of her friends for a car ride, when he drops her off he leans in and kisses her. She tells me and I get upset understandably. About a week later I end up getting kissed by a girl and don't push her off immediately. I tell her and she gets upset.

We get back together all is well.

About a year later we breakup for our third time (second and third time are due to stress from her intimacy problem, work etc.)
and she has the guy that she liked from a year earlier over for drinks, at this stage her brother and friend have gone to get another friend from his house. I come home (I don't live with her but her house is a second home to me) to find he is shirtless and they are on the couch watching a movie. There is a strange mark on her neck. I immediately know what it is. It's a hickey. When I am leaving I confront her about it and she lies to me. The next day I am at a friends party and I ask some of her friends behind her back (she is not at the party) and they confirm it. When I confront her she admits it but is a bit angry I went behind her back, my friends gave her a hard time for kissing someone even though she is single, and I give her a hard time for lieing. I should have defended her but I didn't. About a week later I went clubbing and ended up at a girls house the next morning naked. Obviously I have had sex even though I didn't want to, but the deed is done. My friends don't know where I am and she calls them looking for me so we can hang out. They lie to her saying i'm at there's asleep. And I lie to her agreeing with there story because I am still not sure what exactly happen. I was going to tell her but she found out a few days later before I could and she get's real angry - fair enough I just gave her a hard time for lying and I lied to her face and she does have intimacy troubles. About a week later she gets drunk and has sex with some guy and tells me about it. So we are kinda even on that I guess except I lied. We are both single still. Now she has been hanging out with this guy and she has started to like him because we are going through a rough time and he's there for her, and they have spent the last 4 days together at hers alone because her mum is away. And they have both admitted to each other to liking each other. I love her very much and she loves me.
I'm scared because the way we got together was her ex and her were going through a hard time and she left him for me in one night because it was too hard for her. And i'm scared she will leave me (Technically single) for him because it's getting a bit hard. How do I break her cycle from running away from problems?

I think I deserve a second chance. We have had the times of our lives together and we were so happy. I don't want to let a couple of mistakes ruin what we had. Because I know we can be happy together.
What should I do?

Sorry for the long message I felt it was necessary for the whole story not just a partial one.

[Bonfire]

I know her type, she will run to another guy whenever there is problem. You will not get her to stay completely faithful. I bet she lies to you often but you couldn't tell. Are you sure you want to go down that road because it can get worse when she's in her twenties. I can almost guaranteed she'll cheat on you if you do get her back. She'll definitely run off when there is problems. You can't change her, it's part of her unfortunate upbringing and past abuse. She has to figure this out herself and want to change herself. Whoever dates her will go through what you went through. It will be the same pattern for her with every relationship she has. If you must get her back, then be patient she'll run to you when she has problems with the other guy. I think you are better off without her though.

[Inivisiblewar]

If you knew her though she is not the cheating type. And she is completely honest with me. I know. But her running away from her problems really does bother me. I just wish things could be simpler. We had a bit of a fight tonight and I told her a couple of things which she didn't know. We arn't on real good terms right now but we are both thinking about alot of things. I love her and this is the toughest thing I have ever done. It's making me sick to my core.