I have been following your postings from when I started, because I am so impressed by your gun collection and knowledge of them, so I guess I only know a bit about you from what you post. I really put you a lot older than 20, certainly for what you have achieved in your life, even more amazing that you are so young.
One of the customers in my pub in England used to say to young people he was advising, 'I have been as young as you, but you have never been as old as me'.
I am 52 and can tell you that the best time I had with women was between 30 and 40. Been married and divorced twice, widowed once and now with a lovely lady number 4. Yvsa, was the first to say it and it is true. Regrettably 16 and 17 year olds are like that. They don't know what they want and they aren't mature enough to make the right and honest decisions in life. I realise I am painting with a very broad brush here, but you know what I mean.
I take it that nice house where you live is your parents then? If they leave a room when you enter, pin a note up somewhere and tell them what the girl is like. Hell, print this discussion off. Whatever happens, they will get over it in the same way you will. Clean your guns, go and spray the garden with lead, go to the range, build that fireplace in the basement

but do something constructive, not destructive. You do the unthinkable, she wins, you lose.
Time is a great healer. You hang in there sunshine, 'I have been as young as you, but you have never been as old as me'.
Edited to add: I was drifting off to sleep last night and was still troubled by your predicament. There was something I meant to say and couldn't pinpoint it. Then I remembered... "why she had been sad was because he dumped her".
She was sad because HE dumped her! Not because she had upset you, and you had found out she was unfaithful to you. You have to see that this is a very wrong attitude towards you and a very selfish girl.
Each day will get easier as soon as you say "she is mine no more, I am a free man".