Hey man. Well first let me preface everything I might say with this: Love is extremely complicated. It might be hard for anyone to give you advice about this who doesn't know both you and her very well. Do you have any compassionate friends who aren't stuck on themselves or phony and putting up a front? Perhaps your brother? Hell even your Dad if you have a relationship with him like that. But you posting this here, I'm guessing for whatever reason you don't have those kind of advisors or maybe they're not saying what you want to hear. Whatever the case here is what I've got to offer.
As a general rule any girl who would be physically intimate with you while still in a relationship with another guy runs a good chance of also cheating on you some day. Again though, I know absolutely nothing about her. And to be honest, one of the best relationships of my life began under the same circumstances.
I understand she is not happy with him and he is leaving, still infidelity is never right. She should have either broken up with him before allowing herself to do that with you (and yes I feel infidelity is not just sex, its any romantic intimacy) or held off with you until he was gone. I say should though, we live in the real world don't we. I've been there man, believe me I'm judging no one. Love is the strongest thing I've ever seen, if you can control it with will power then I don't think its love anyway.
As to the question of what you need to do to prove your love. Hmmm? Well first let me direct you to a passage from the Bible: I Corinthians chap. 13. Especially verse 4, but the whole chapter really. "Love is patient and kind; love is not jealous or boastful; it is not arrogant or rude." So on and so on, you gotta read the whole thing for the full effect. Don't get me wrong now, I am by no stretch a bible thumper. I am in fact a strict agnostic which means literally 'one who does not know'. I want to know though so while I can endorse no religious veiw I can discount none either so I read as many religious texts as I can and there is some profound wisdom there. I'm rambling aren't I. I say this only because I know a lot of people who won't even read the Bible because of an unfair stigma they placed on it. That is stupid and closed minded (I hope any one reading this is not one of those people but, I'm sorrry, it is stupid). I highly recommend reading that passage, it may give you a better handle on the subject of Love.
You're right too. Saying you love them and being there for them may work for a pet dog. Feed 'em, walk 'em and buy 'em a toy and you've got a devoted companion for life. With people its not so simple I guess (although maybe dogs have it right, dog is God spelled backwards
). Seriously though I don't think anyone can tell you exactly what to do to prove love to someone. Stop thinking about what you should do and just do what you feel. If you do it out of sincere love she's gonna know. Anybody can send her flowers or whatever. How many guys could hold her for hours and stare into her eyes and tell her how beautiful she is and mean it. Just follow your instincts. When it occurs to you to say you love her, say it; when it occurs to you to tell her how gorgeous she is, say it; when it occurs to you to make love to her on the beach at sunset, do it. The point is, mean it when you do it and she will know, trust me. Expensive jewelery and stuff will give her a thrill but to really be loved; well that's the highest high in the world. Once she has felt that, all the questions are answered and the path is clear.
I understand though. I see you're 15, and at 24 I am by no means the wise man on the mountain. Man I would give anything to recapture that feeling of being in love when I was 15, its the greatest feeling I've ever had. But I was unsure too. I didn't know what to do either. I know now that I did know what to do I just wasn't sure it was right, so I didn't really do anything and lost. Its like; imagine your standing in front of a time bomb and only have a couple of minutes to do something about it. What can you do? You don't know what wire to cut to stop it and you don't know if you can run far enough to save yourself. So you can either run, stand there and run out of time trying to figure out what to do, or pick a wire and cut the damn thing. Except its a little different with love, you already know what to do. You know what wire to cut but you still doubt it. Are you afraid of showing your love and being rejected and feeling humiliated or something? Man fear is gonna get you nothing but regret. A mistake at least gets you experience. From what you've said though I don't think you even have to worry about that. I'm not gonna say it anymore after this okay, I know its getting redundant, do what you feel. All the things you think about saying to her when you're alone but don't, say them. All the romantic fantasies you have when you think about her, make them real.
Well anyway, sorry to write you a book here but good topic man. I don't know you, but being in love is awsome and everyone deserves it so if I can help I'm happy too. Good luck. Hope something I've said helps you. Let us know how it goes okay.
BTW how is Germany? Are you a native German? I took German in school for a few years, nice culture. I live on the beach in the states. Greetings to you from the surfer dudes