Why in the hell do so many people think they can be in a committed relationship and fuck other people without creating jealousy, insecurity and resentment?

The vast majority of the time this shit never works out. Don't listen to people telling you how great open relationships are. They almost always end badly.

Be an adult and pick a partner who you like and are compatible with and stick with that person. If you start developing feelings for someone else then either cut that person out of your life if you are committed to your current partner, or end it with your current partner and go be with the new person.