I'm upset that my girlfriend made plans with another guy on valentine's weekend
Why did your girlfriend make plans with someone else on Valentine's weekend?
She only thinks about herself and not about my feelings.
I'm sure you're really frustrated that your girlfriend plans to spend time with another man on Valentine's weekend. Tell her how you feel and explain the situation. If she refuses to change her plans, you may want to re-evaluate your relationship.
I'm not going to allow her to spend time with that other guy. I will make sure she stays home alone.
It will make the situation worse. You better talk to her about your feelings and definitely your anger is justifiable but you need to have a conversation with your gf. There is not point in forcing her into doing what you want.