You don’t need to feel guilty about your friend becoming your boyfriend. You two met first. He handled it respectfully and didn’t lead her on. He never thought he’d have you so he started planning his life without you but you did the best thing you could and let him know. And he was a man about it and acted on it straight away. That’s how you know he LOVES you.
I am in a similar situation where I settled for my wife, but in reality, I’m still in love with my childhood friend. It was only after marriage that I got the sense she had wanted to confess to me, but she never did. I wish she’d had your courage and said something. If she showed up at my door tomorrow and told me she wanted to be with me, I reckon I’d leave my wife to be with her. She’s all that matters to me.
I wish you two a happy life together. At least there’s one couple out there following their hearts.