Well i am no casanova, but have had some good and bad relationships, but i feel that what you need in this case with this girl, is for you to be Secure in your Manhood..if you are unsure of that..then do not get involved with her or any woman until you are mentally ready to accept yourself and how you can accept or reject thoughts of infidelity. If you go into this with negative thoughts that she is fooling you or fooling around AKA..Playing you...then you are setting yourself up for failure. good luck..and take your time..old saying...there are plenty of fish in the sea...
