well...if u think it carefully, the only way you could do is.....tell her directly that u want to break up with her..and better tell her the reason....coz it s the only way to do it....
no matter how u beautify the way to break up....it hurts your gf.....
but before you say break up...i advise you to rethink seriously and ask for some assitance....there is some counselling before marriage in Breakthrough.....i think if after counselling, you still find that break up is the best way, i believe your gf will be more calm and may have the same feeling too.....
So do find assistance before break up, especially u guys have planned to get married....dont do something that make yourself regret for life