Aw, man, that's some deep shit. I know what it's like to have my life, for the most part, ruined by a girl (Yuki knows this intimately... I've talked to her).
Life's a fickle thing, my friend. And love is even fickler. My fundamental philosophy of love is this one sentence: it is a risk and a sacrifice thereof.
Never regret anything you have done (I have.. and it hurts sometimes more than the initial pain). Each moment of your past forges your present and future and creates who you are. If you wish you could go back to change something, that'll change who you are completely in this time.
My fundamental philosophy for pain therein, is that: it doesn't matter how pain comes, why it comes, how deep it is, or how long it lasts. What matters is how we react to it, and our reaction shows the whole world who we are.
I can't tell you how you should live your life from this, the choice is yours. This life, this world will only ever be what you make of it.