Guys help: who would i chose a person that i've known for so long or a person that i've known for only 6months?

here's the story:i'm a guy and the first person is my long tym gf. who would i pick i both love them. when im with the second person i felt so good like everything is magical. we have a lot of pettings and i knew the way she looks at me, she likes me. but why am i feeling this way i also love my gf but i felt more in love with the other girl.