Allright, I recognize that you are in the painful rejection and self-doubt phase of this, but don't go down the road of self-pity or self-character assassination. Though it doesn't feel good at all, this is an incredible opportunity for learning more about yourself and more about relationships in general.
First and foremost no one is ever in a relationship by themselves and there is plenty of blame to go around. It's never 100% one person's fault that things take a turn for the worst. Both people in the relationship have to be willing to look at themselves and focus on taking responsibility for how they can be a better partner.
So, with that said, let's look at this "you weren't there for me" stuff. Comments like this are often made when someone in a relationship is more focused on themselves rather than their partner. Additionally, a statement like this can be a sign of poor communication. I get that you said the two of you "talked all the time", but simply talking about stuff and actually communicating feelings, hopes, and desires are two entirely different things.
Genuine intimacy comes when we are willing to be vulnerable with our partners and that means communicating our greatest needs despite our fear of rejection. From what you have described you were clueless about her feelings because she didn't clearly express them to you. "I thought I always knew when she was upset or sad" were your exact words. That tells me that you were in the common position of trying to guess or figure out what was going on with your partner as opposed to the two of you having reached the point where you can openly and honestly express when you are upset or feeling disconnected. Relationships with lack of communication, mind games, or manipulation are generally doomed for failure. If she wasn't willing to openly communicate with you but, instead wanted you to intuit or pick up on signs then you need to recognize that those kinds of action are unhealthy.
Mourning the loss of this relationship will take time, but spend that time wisely by seeking what you can learn from it. Explore what you want in your next relationship. What characteristics do you want in your next partner and how can you seek to be the best partner you can be? This kind of wisdom often only comes from life experience and that's where you are right now.
All the best and let me know if I can help out in any other way.