The email your GF sent you is not new. It's the same email she sends you, the same message she gives you repeatedly. She is not ready, she wants to take it slow but wants you to hang around until she's in a better place. Your reply is the same as in the past.
You push for a future, a girlfriend, to be a bona fide couple. She's not ready. You accede to her wishes. The whole ball of wax starts over again.
I think she wants to be a Good Friend. If you care for her, reread her email without intentions or expectations. Reread her email and see where you are in this picture. Consider what advice you'd give a friend in the same situation.
And for goodness sakes, please stop beating yourself up for learning as you go.
Me.. I'd say be her friend, remove any and all romantic entanglements, drop her as your emotional confidante (if she is one). And take TIME for YOU because you seem to find satisfaction in a relationship that is not there. Like Gucci said.. if she wanted you closer, she'd make it easy.
And from my experience, if the guy is right, he is right. Doesn't matter if there's a good enough guy in the wings. The guy with 'something there' is irresistible.
Talk to your counselor, friends why you are so insistent on pursuing a woman whose wants and needs are counter to yours. Why you cling to hope. Holding onto the belief in marriage and family is one thing. I just get the feeling you're not a happy camper emotionally and having something, even if it's unsatisfying, is better than nothing.
Look at yourself, Rob. Just be Rob. Heal, learn and grow, one step at a time.