There's a reason it never worked out with anyone else.

 When you jump into bed with his best friend, stay there. Don't get out simply for the sake of "doing the right thing." You'll be left at the altar anyway and the best man won't be there to save you.

 Love her. Kiss her. Cherish her. Don't take your time for granted, and don't piss away the opportunities being thrown in your face. You'll regret it.

 I want to say "Don't marry her", but if you don't, you won't have those 2 really wonderful kids. You decide(ed).

 Don't make your whole life about him. You have much more to live for than a guy.

 She will always be the favourite - Daddy's girl, the popular one, the beautiful one. You're the studious, adventurous, courageous, independent one. You always wanted to be her. She'll never tell you, but she wishes she were more like you.

 It'll be the most awkward kiss of your life, but it she turns out to be the best girlfriend ever :)

 Your illness is actually a blessing in disguise. Don't lose hope.