Bright, I'm sorry you were feeling badly yesterday, but today is a new day and hopefully you are feeling better. The age is just a number. Card and gift stores have really put the focus on the numbers, i.e., 40, 50, etc. People, many years ago, didn't put as much focus on the age of people. Fifty is just a number and let me tell you, people are going strong at that age and it's nothing to get down and out about. I just recently noticed that there is a new online dating service for people over 50 and it's doing quite well. I know of many people who have lost spouses and have found love again after 50, so please, don't knock yourself because when you least expect it, someone will come into your life...but you aren't ready for that just yet.
As for your GF, you need to start politely cutting off her conversations about her problems. It appears that the only time she calls you is when she needs a sounding board. It's time to give her a bit of her own medicine. A friend is someone that listens and no matter what you are going through is there for you. True, you may be going in circles, but that doesn't mean she gets to be the judge of what you do. So, my advice, gently change the subject when you speak to her, especially when she's calling just for a vent and/or support.
As I have pointed out many times, people do not know what you are experiencing and until they walk a mile in your shoes, they will not understand. People tend to be relieved and happy when we have finally moved on because they do not want to see us unhappy and/or stuck. They want us to live our lives to the fullest.
You will develop a tougher skin about such things as you continue your journey...but you are the only one that knows what you are going through and we can't "assume" that others will understand, especially when we've been at this journey for quite some time. They do not understand that this is not a normal separation/divorce scenario, so it stands to reason that they would think that we should have moved on months/years ago. Don't beat yourself up over what others thinks...you have to take care of you...you'll know when you've had enough and want to cut the ties and move on w/your life. Until then, live your life to the fullest and do what you need to do to survive.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.