...what was your biggest mistake in your current relationship?..

In my opinion, pre-marriage relationship (you know, the boyfriend and girlfriend thing) is a good place to test drive your mental, faith, and trust. It is also a good place to determine which person is good for you. Why I explained it first? That's because it will be the background of my answer. First, about your case; What kind of commitment you've made with your girlfriend? Fun relationship, or a serious one (will go to marriage)? If the first option, then it is actually okay, but you should be honest with her, so that she can choose to keep going along with you or leave (that's because the reason is still fun relationship, so no hard feelings there). But if you are commiting serious relationship, dating another person is sure a mistake which you should never repeat again (you've learned, right?). As for my mistake, I've done the above, but for the first option.

I don't like guys who doesn't know what commitment is all about.., if you were in your girlfriend's shoes.., and she will date some other guys while she is still committed to you.. would you like that? not.. right? I don't want to sound judgmental here.., But6 i just want you to know.., that what you did is not good., and i know you are aware of that.,., pls don't do it again.., =)