I truly feel for you, OP, children moving out leaves a horrible void and, most of all, is the end of true "mothering", as it is the final sign that our little ones are finally independent adults who can function perfectly well without us.
The fact that you brought them up alone makes it even harder. When my children left home I was able to talk about the emptiness with my DH, who felt the same, and reminisce about the childhood years. It is actually like a grieving process that you have to go through for the loss of your status as lynchpin of your DC's lives.
Just because they don't need you on a daily basis doesn't mean they'll never need you again, or include you in their lives again. The only way to go is to avoid moaning to them, be cheerful and positive with them even if you don't feel like it, so that your relationship is enjoyable and they will call you and visit you because they want to, and not because they feel a guilty duty to do so.
Your youngest is still caught up in the heady excitement of leaving home and being completely free, but once the novelty wears off a bit he''ll be in touch regularly, I expect. Try not to make too many demands on him, or on your eldest, as unfortunately, neediness tends to drive people away 
Most of all, don't feel you''ve wasted the past 23 years - you have raised two boys to adulthood single-handed (which is something I respect very much as raising children is no easy task) and what they are today is largely due to your efforts. In time you will realise that their leaving is just the normal course of life, and if they were unable to detach themselves from you it would mean there was a problem.