Can you give us some examples of why you think you might be a bad girlfriend.
Here’s a list of some actions that I think make for a good girlfriend or boyfriend: being kind and compassionate, being generous with your time and assets, being a good listener (and not being a nag) complimenting your mate when he or she has done something helpful/kind/romantic, pampering your mate (not at your own expense, and not without getting pampering in return, although in most cases one person gives more than the other) being loyal to your mate and watching his back, not cheating (nor giving the slightest appearance of cheating) and this goes for emotional cheating as well as physical cheating, being there for your mate (not being so independent that you never need/want him) being a partner to your mate (you actively do things together as a team, and you give each other enough space to enjoy things separately without feeling jealous or lonely) finding out and really knowing what is important to your mate (and what is not kosher) and then making a point to acknowledge those things and implement them as best you can, knowing that 2 people are not attached at the hip and have different likes and dislikes and needs and respecting that (rather than trying to change the other person) doing little kindnesses out of the blue (instead of having to be asked repeatedly to do them, or having to be reminded to do them) being kind to his family and friends (even if you don’t like them, short of these people doing illegal or abusive things, some people just don’t mesh well, but you got to be kind to them anyway for the sake of your mate) letting your mate know by your words and your deeds (not just one or the other, and not just assuming he knows) that you love him, not being passive aggressive, not being outwardly aggressive, and never call him names (even if you’re very angry) and by not ignoring his interests (even if you think they are boring).
Those were the qualities that I came up with just off the top of my head. Think about these things and see if you do most of them, and if there are some that you don’t do, then we can work with that, to figure out why you don’t do those things.
I suspect that you’re a better girlfriend than you think you are. Maybe you’re just having some depressive low esteem feelings. Let’s figure it out together : )