I'm going to hang out with my ex without telling my girlfriend
That feels very wrong. Why would you spend time with your ex and not tell your current girlfriend?
My ex makes me happy for a couple of hours
It looks wrong that you are hanging out with your ex. You should be honest with your current partner about something like this.
So I should feel bad if I want to hang out with my ex girlfriend? Since when do you police my life? It was just an hour , cut me some slack.
The issue is that you're hiding it from your current girlfriend who may be hurt seeing your ex with you and may not approve of your relationship with her. If you want to be with your ex-girlfriend, then you should break up properly with your current one if you must. You can't have two relationships at the same time.