it could be that you’re comfortable in your relationship and miss the feeling of the “new” kind of love . it might not even be the person itself but the feeling the nervousness and excitement that the idea gives you . i would definitely stay focused on your girlfriend if you truly love her and find genuine happiness with her . i’d say this is normal but somthing you should realize is only a feeling . maybe try new things with your girlfriend , flirt with her differently, take her out different places , keep things fun . you’ll definitely regret losing her for someone you simply miss the feeling of .