hello ampm, so sorry you are in such a sad situation.
I am not surprised this has hit you so hard - not only was it one of the foundations on which you built your life after the shock of being widowed so young, but you have now suffered actual rejection at a time when rejection comes hard. As you say, you face your daughter moving away, and however much pride you have in her college attendance, and her growing independence it is a big change in your staus as mother - and you will lose the comfort of a day to day friend in the house. Also, you must be approaching peri-menopause, which may well be making you feel fragile emotionally, and signals another change in your life - however matter of fact you are about it.
You know, you have so much to be proud of: you DID support your daughter after the loss of her dad, and made life go on. You DID create a happy and succesful relationship, which, while it was mutually good was an achievement. You are the mother of a girl poised to make a place for herself in the wold, you have energy, vision, a job....be proud of yourself, and know that you can move on not merely as an abject survivor of this, but to new and more challenging things.
Birthdays, shmirthdays. Do something ordinary - go to the cinema with one good friend, and for a curry.
And choose one thing - simple or complicated - that you would like to do for yourself and have never done. A place to visit, a course to take, and experience to try. Something that will give you a sense of achievement. (bungee jump, volunteering half a day a week, aerobics, swiss ball, a language course, visit a gallery in another town and find out about an artist you have never heard of...hatever) And just do it. You have no need to be looking forward to growing old at your vigorous stage in life - be looking for the next phase.
I am not underestimating the pain of rejection, but (IMO of a relationship breakdown at a mature age) it is easy to imagine that all your eggs were in one basket, and to feel that your life cannot be anything again - and it is importan to give yourself little reminders that this is certainly not the case.