So this guy is hitting on my girlfriend with the PUA techniques,she doesn't know about it and i can't tell her about it.
She says that they are very good friends.
But I know what he is doing.
I love her and she is the 1st girl in my life that i have been completely honest and open,can't afford to lose her.
how the fuck am I supposed to stop that guy? / what should I do about it?
P.N - she is very innocent and childish at sometimes and very impulsive.

Lol. The only way it will work is if she's unhappy with you or she's a cheat in general. Is she unhappy? Probably not. I wouldn't worry about it but I wouldn't be a pussy. If he starts asking your girl out on dates or starts trying to ramp things up then square up to him.

And what techniques is he using anyway? Is he just now using them or has he always been like it? If he's just now using them and you genuinely think he's into pick up then square up to him in person. If he's always been like that around her I wouldn't worry too much unless he's definitely trying to go all the way with her.

But general advice is to be the non-caring alpha bro and let your chick make up her own mind which is exactly what you should do unless this "pick up artist" is going all out to try snatch your girl in which case you have every right to square up to him and tell your girlfriend you don't like what he's doing with her(Well, depending on the severity -- It has to be within reason or you're sen as a control freak).

A few weeks ago they were talking on fb and it seemed she was interested in him,it was slightly-flirty from her side,she doesn't know how to flirt(I have seen her texts)
But later she told me about it and was VERY sorry and cried like for two days ,
she tells me everything,she doesn't lie to me(once or twice she did and after an instant she apolozised and confessed about it)
she insulted her badly,but this asshole keeps trying and it bothers me like shit sometimes
She told him not to flirt with her (She is too innocent to understand that he will still try)
and they talk like friends now(That's what she said and i trust her)
She is happy with me(I know it)
If i tell her not to talk with that guy she will be more interested in him,she will stop talking if i tell her not to but i do realize that she is hot and many guys are gonna hit on her and she is gonna have to deal with it
And how do i square him?
P.S -Nobody has touched her before me ,she is 17, and i turn her on
i have been there for her emotionally as well as physically
i am 19 and she says it's a big deal for her that i 'touch' her
and she loves it
and environment affects her,she really cares what other thinks about her and gets easily fucked up if anyone tells anything about her and enjoys attention
she has low self esteem and gets really insecure if i even look at any other girl

Give us examples of what he's saying or doing.

Odd that she'd cry over talking to a guy. She knows she did wrong and is either acting up to get pity points by crying(Women are notorious for this) or she went further and the guilt set in.

Been there, done that. A guy was coming after me and my boyfriend (at the time) went full alpha male on him. He told me to cut contact with the guy and never see him again but it was exactly that attitude that pushed me away more. Eventually, after a lot of coercing, I cheated (and confessed to it) and the relationship deteriorated.

Be protective, but not possessive. Let her know that she's special and make her feel like she doesn't need to go to whoever else this guy is to get the attention/appreciation she's looking for. Let her know about the PUA tricks he's using (without letting on that you use them yourself) and that they're standard shit he's just been reading out of a book. Most importantly, let her know you're there for her and that if the guy makes her uncomfortable, you'll help ward him off.

Although the advice is a little against the 'abundance mentality' culture of this forum, it'll help you keep your girlfriend.

Jeffy, are you a guy or a girl?

you meant full beta male, right?

_________________
You must be overconfident and cocksure, even if you haven't got a god damn thing in the world going for you. And you must fail with women until you do not fear the possibility of failure, whereupon you will succeed wildly.

that douche is making her jealous with some other girl(her enemy);tells her he will do this ,do that,go to places,invite her over family dinner
acting like she is already her gf
trying to act all dominating and shit
it's fucking killing me
this pressure is so much
what should i do?

For the millionth time. What exactly is he saying and doing. EXACTLY. Stop giving vague bullshit replies that gives ZERO perspective as to what it is that's going on. If you want opinions you have to give more information!

I'm a girl. Does it matter?

OP, your 'it's killing me' attitude isn't helping your situation. If you get too paranoid, your girlfriend will be able to tell, regardless of how innocent or naive she might be.