In my opinion, there are two ways to go about this:
You mentioned that you haven't had a chance to explore your physical/emotional needs yet. After getting into a relationship, do you feel any less attracted to your girlfriend? When she offers you her time and affection, are you able to receive it well? Is making you feel loved and appreciated in a way that you do not feel apprehensive about it? Are you also able to reciprocate this without feeling distant/detached from her? Sometimes we can see the people around us trying to be loving, but we just can't perceive it as true love, because the way it's conveyed just doesn't work for us.
If not, then perhaps you should take a break from the relationship to find out more about yourself. She sounds lovely, but by having such doubts you are hurting yourself and her, instead, gently discuss this with her and then spend some time with yourself. What I've found is that you can never get into a relationship because you're lonely. You must first become familiar/comfortable with yourself and this will teach you many things about what type of affection makes you feel the most loved. Just because your girlfriend loves you doesn't mean she is the only one who will ever love you, so it doesn't hurt to be single for a little longer to end up with the right person.
HOWEVER
What otherwise may be happening is that this is the first relationship where you are receiving deep and unconditional love, and this may feel unfamiliar. This is not a bad thing btw, it happens when we haven't been treated the best in the past, we feel we're only worthy of more conditional/inconsistent relationships. Sometimes the "missing out" feeling comes from not feeling the uncertainty and anxious attachment that may be present in those relationships. In this case, remind yourself that everyone deserves someone who loves them deeply, and you and your girlfriend can be that for each other. Try to enjoy her love rather than push it away, and see how that makes you feel.
Your sister may also be playing a role in these emotions. If she's someone whose opinion you value, then hearing that you can do better may be causing some inner conflict between what you know and what you're hearing from others. Even if her opinion didn't cause this conflict, it's most likely fueling it. Although she is your family and she is trying to show she cares, it might be best to set some boundaries with her, if her comments are regarding your girlfriend's appearance. If they are regarding her personality, then may you should ask your sister what she sees as her negative traits, and perhaps this may notify you of a trait you haven't noticed before.