Hi need-to-know,
I am sorry to hear that you are struggling with doubts about your relationship. I may not be able to provide the advice or support you need but there are actually quite a few others who have been/are going through something similar to yourself. Let me link their threads below.
http://www.uncommonforum.com/viewtopic.php?t=58690http://www.uncommonforum.com/viewtopic.php?t=104930http://www.uncommonforum.com/viewtopic.php?t=105003All of these people have slightly different situations, but the theme is the same. They all have some sort of relationship anxiety. I hope that reading about what they have said can help you, and you may even be able to contact some of them. There are more on this forum, but I can only link three...
From what you have written it sounds like you really care about your girlfriend, she is supportive of you and she is a genuinely nice person who you enjoy spending time with. I would say for those reasons she is someone worth keeping hold of. Don’t let those doubtful words echo in your mind. Instead, enjoy your time with her.