I'm with you Cat, this did not allow your "monogamous" girlfriend time to deal with the situation. I feel for the girl that you heartlessly dumped on Valentine's Day. Yes she overreacted but you treated her in a very unfeeling and disrespectful manner. First you CHEAT on her, then you tell her that she has to get used to it and then you dump her on a day that many women think is the most romantic day of the year? how did you expect her to react??
Did you give her support when she was feeling jealous and try to help her work through her jealousy? this woman was feeling insecure and used in your relationship (I am assuming this as I don't know the whole situation) and felt like she wasn't getting any support from a person who she thought was a support in her life. She lost it. She cares about you and didn't want to lose you but it sounds like she had no way of telling you that without you thinking that she was being irrationally jealous. Next time you do this to someone, and they tell you that they are feeling jealous, give them some time and support to accept those feelings. Jealousy is a normal human reaction to feelings of insecurity, people are going to feel it, please don't demonize this girl because you didn't support her.