Yes, completely.
If you feel alone, please don't! Not only has this made me hesitant to settle down with that special girl, but has also made me never really able to trust previous girlfriends that I have had for the fear of this exact reason. I personally try to stay away from girls with boyfriends maybe just because I am a compassionate guy and know this fear, but I know that there will always be guys who don't give a shit. I can't say that I can give you an answer on how to get over or deal with this but I can say that I feel the same way you do about this situation
