He may be more available in a few years time, but would you really want to put yourself on hold for all this time just in case? And what if this wasn't the whole reason he ended things and wouldn't want to try again anyway?
You say that you'd be more happy if you were his girlfriend, but I doubt it would last long. I mean, if he can't fit you into his schedule, what's the point? I imagine it would end with you feeling dissatisfied and unhappy. And his decision about not putting you in this position isn't just about him thinking of you; he probably doesn't want to deal with a girlfriend who isn't happy with a situation and wants what he can't give.
In the meantime, I think you're doing yourself a disservice by staying 'best friends' with him. How is either of you going to move on and find new love when you've got a best friend of the opposite sex? No future partner will want you if you've got another man as your best friend. And likewise, he will have the same problem with other girls.
It's time to get realistic. Let him fade from your life and move on to someone who has time and love for you.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.