True demon took the words out of my mouth!
The story sounded highly suspicious before and now this cements it. Plus she's 18. Can you really trust an 18yr old girl? This girl cheated on you at a party and is flirting with a 15 yr old guy. Really??!?!? If she's even entertaining 15 yr olds she's blowing 21 yr olds. Dump her.

Alright - an update:

Me and her chillaxing in bed, watching tv, i suddenly get a text and she goes uber-jealous mode.

Go pick up your phone, text back, it's annoying if you dont text back. I say i cant be arsed to text back and she just shut me off, moving me aside and saying You're not allowed to touch me if you dont text back. Jealous as fuck.

I just stood up, packed my shit, didn't say a word, gave her 10 minutes and went back to her saying.

girl, this aint working for me, I'm gonna be leaving, this shithole feels more like frustration rather then a relationship. I was gonna invest big time but i realise it just aint gonna be working this way, take care.

Left and went for a walk before i got picked up.

I'm not feeling that bad allthough i probably will the next few days, i'm probably gonna be getting a text tommorow, gonna share some of my thoughts and put an end to it.

I litterally have changed from the most beautiful lifestyle to something corrupted. I went out, sarged a couple of days per week, had so much fun picking up girls, had so much succes and i was having a blast. I meet this girl , i fall in love, i reflect myself and i cant find the ''me'' back. I litterally have changed from ''Happiest guy alive'' to ''Sentimental piece of thrash''.

Actually, i got a text aswell, it's a long one and this is what she says.

Attempt to word some of my thoughts in the moments of silence: At every word you say thousands of thoughts flash around in my mind. As you can guess, the thoughts aren't positive, i put everything at doubt, life just isn't a beautiful fairytail, atleast mine isn't. I would wanna do all the nice things you say we'd do, but they just sound to good to be true. I know i've been very distant with you lately, you'd do the same if you were in my shoes, If i get to attached to something, it's the way i act, because at some point, you'll lose it all anyway. Not the best way of handling things, i know... But i really dont know what else I can do. By shutting down and not telling you anything, i can hide my feelings and i wont collapse in tears. You asked what i wanted? Read your letter (She wrote something really sentimental). I didn't really have a nice day, well he was fun and all (She went to see her dad, her parents got divorced) but it was hard seeing my dad so happy with his new wife and that i dont have it that way (she has a stepdad she cant really rely on despite him being really awesome, probably because she just doesn't wanna accept him). I dont want you to text me back because i dont wanna hear more bad news.

Despite all this and how truthfully she sounds, i still doubt her the same way i did before, it's probably a momentum because she realises i'm going to leave her and she doesn't wanna lose it. I still dont trust her not do i know if i ever can. I think it'll be going well again for 2-3 weeks before shit hits the fan.

Good man! Of course messages like these are going to be hard on you and might leave you feeling guilty. Her reasons sound plausible, and you can't blame her for her attitude. After all her role models (parents) showed her that nothing's forever. So I can totally understand where she's coming from...

That's why I'd like to point out what had been said in this forum a number of times: If you consider someone to be your girlfriend, check the relationship with her parents. Especially with her dad! It might be "cruel" to say, but based on my own experience, I might not want to ever date a girl who's seen her parents separating, at least not if it happened in her younger days.

Even if you still care about her, it should be all about you right now. Go out and have fun!
Actions ALWAYS speak louder than words, and she has not behaved well at all.

Staying with her sounds like it would be an emotional nightmare that would leave you second guessing everything she says and does and she'd be hot and cold, jealousy games, all that shit.

She's 18, don't get attached.

Yeah, it's definatly tough but I'm glad for all the experience i'm getting out of this situation. I just cant get myself thinking of continue'ing with her because i know it'll just go well for a week or 2 and when things get more stabilized, shit will again be the same, distant, different attitude, ...
She's really emotional, she cant think positive and she cant get it over her heart to trust me. I cant be myself around her. I know that eventually strange things will happen, she will reflect her feelings in a bad way, etc...

This right here sums it up you need to end it NOW or else shes going to convince you to stay, go out and get a crap ton of numbers and dump this train wreck whose probably getting with 15 year old guys