I am concerned by the questions you asked me (am I overreacting, smothering her, not understanding the impact of the age difference, etc. ) What you are doing to yourself is called mind fornication (this is a polite version of the more popular expression that I am sure you have heard of). This means that you are raping your own psyche, trying to talk yourself out of what you feel and what you know. In short, this woman isn't being responsive to your needs and doesn't return your affection. The reasons don't matter. You aren't being treated the way you want.
So, it's up to you do examine yourself and figure out why you are attached to someone who isn't treating you the way you need to be treated. Are you replaying a familiar scenario from your childhood in which you loved a parent who didn't return your affection? The fact that you are so quick to make allowances for her and suck up your suffering makes me think that you are no stranger to this type of treatment. Just as you likely made excuses for your parent or parents' neglect of you, you are making excuses for hers.
Keep in mind that all battered and neglected kids deny their anger toward their parents and resist pointing the finger at them. This is called the narcisstic defense of childhood. What this fancy term means is that abused kids guard their parents from their own anger (and often turn the anger back on themselves rather than direct the anger at their parents). Abused children do this for several reasons: 1) they think that their angry thoughts can actually destroy their parents, and since they need their parents to survive, they can't allow themselves to feel angry at their parents; 2) all children think that they are the center of the universe and that everything good or bad that happens is their own doing or their own fault. This means that if a child is abused, he figures that he deserves it and blames himself not the parent; and last but not least, the abused child continually tries to 'be good, ' hoping to be rewarded one day with his/her parents' love. If you read over your letter to me, you will see how much you sound like an abused child who refuses to point the finger at this parent (or girlfriend). and blames himself instead (you all but say the the problem is your fault, and due to your neediness, etc. ) In reality, the problem lies in the fact that you keep rewarding her neglectful behavior by continuing to give her your affection.
What you need to get is the fact that the more you reward someone who mistreats or neglects you the worse that person's behavior becomes. She reads your continued affection as saying, 'Go ahead dump on me. It's all right. I'll still be good to you. ' If you want to receive better treatment from her, you have two choices: either tell her straight out how you expect to be treated by her. If she refuses to respond, then you know that she is too damaged to give to another person and it's up to you decide if you want to continue the neglect, knowing full-well that she isn't willing or capable of change.
The other way of handling the situation is to stop rewarding her neglectful behavior. Pull back, pay her less attention, let her call and chase you. If she doesn't respond more favorably to you in this barren atmosphere, then she probably isn't capable of giving to another person, in which case you need to decide whether you want to stay and keep trying to change her (which will be a useless effort, since only she can change herself) or whether you want to move on. In any case, you can't continue abusing yourself by denying your emotional reality, not unless you wish to continue reliving what was surely a childhood of emotional neglect. Let me know how you make out.