I perfectly understand your situation. There are several reasons behind this kind of feeling such as –
- Insecurities – Click to read more about insecurities.
- Possessiveness – Click to read more about possessiveness.
- Jealousy – Click to read more about jealousy.
- Inferiority complex – Click to read more about inferiority complex.
You need to understand that nor you can force anyone to love you or neither you can stop anyone from loving someone.
When you love someone, you love that person because you see something common. Now, it is not your fault, if that other person sees nothing similar.
If your gf is talking to someone else and this is what disturbing you, I suggest, you should discuss this trouble of yours with your gf. I know, she is going to tell you the same that I just said (insecurities, possessiveness, jealousy, inferiority complex). But, if she is really concerned about her relationship with you, she will give you a good answer.
Please, talk to your girlfriend.