I knocked up my immigrant poor girlfriend in high school and abandoned her which resulted in a baby out of wedlock
Have you told your fiance about her?
I really want to marry her...I sure hope she'll understand
While being young at the time is understandable that you were not ready to be a father, you are no longer that child and should understand that you can't leave your kid like that. Your fiance might not understand your actions and cancel the wedding.
Damn I deserve a punishment for not telling her.
You don't need to continually beat yourself up about it, as long as you are honest to your fiance about this all! The two of you deserve a happy life, despite your past and other child. She would appreciate your honesty about something so serious.