wah piang eh. if i am your galfren and i find out, your balls
are gonna be so crushed that you will beg me to drill you a new
hole instead 
it is up to you whether or not you wan to tell your gal. once you
start having this guilt, it will affect your relationship in a bad
way. if you decided to tell her the truth, be prepared for the
worst.
by the way, please introduce your galfren to sgforums before you
confess. at least she can pour out all her hurt here and have folks
who can help her. and i not refering to you a**es who just wanna
take advantage of her. there are some here who are genuine people
who would lend her a shoulder to cry on and maybe a parang to chop
your head off.