I'm not sure what you're expecting of us here, but here is my input FWIW.
Sometimes a person has to make a decision for their own well-being and the most mature and honorable way to respond is to appreciate their honesty and then pick up the pieces of your life and move forward. Especially if she was born in the first week of Capricorn, this is an extremely intense and potentially scary time for her--there is a Pluto transit looming, already having begun and then retrograded a few degrees. In a few weeks it will begin its full force steamroll over every Capricorn on the planet, one by one. Even those who don't understand the astrology of this are feeling the effects. I have been planning for this over the past TEN YEARS. One of my recent decisions was to quit a second job that was interfering with my personal growth and emotional health too much. And what i found was that it was not as easy as i thought it would be to just respectfully give a month's notice that i could no longer be tied to the saturday shift--the owners acted like clingy, emo, resentful boyfriends and went and told everyone that i abandoned them when they needed me the most. This is exactly what i was trying to avoid by leaving there, and it's why i gave them so much advance notice to find someone else. I don't know if other early Caps are feeling this the way i am, but for me, it's been imperative that i have no attachments other than the responsibility of caring for my children. If i were dating someone right now who wanted more than a casual, occasional and mellow fling with no expectations, i would give him his walking papers too.

You went through this Pluto transit too, depending upon what degree your Sag. sun is, sometime in the past 21 years. It is probably a different experience for a Sag. than a Capricorn but the theme is the same: death and rebirth. It's about letting go of what no longer serves you, whether that be a job, a person, a home, material objects, ideologies or religious beliefs, and it usually comes along with losing some family or friends to the grim reaper. This is all what i've been preparing for by doing some major spring cleaning in advance.

So i hope you weren't coming here expecting a pity party, go to the Pisces or Cancer tribe for that. What would help the most is for you to have some compassion for what brought your GF to this decision, as you probably did similar harsh or selfish things during your Pluto transit. We all instinctively know what's best for us. I'm sorry that her decision left you on your ass, but you have legs, right? Hoist that ass back up then! That's what Capricorns do after we get knocked down.