Hi sticky
Don't he hard on yourself. Everyone has a moment like this.
What I would focus on going forward is the fact that your children are all becoming independent so you will have time to find the new you (and in time potentially a new partner to complete and enrich your life).
One thing you will have that many others in these positions (with young children) is time to find the new you.
Take up a hobby that you have always fancied, reinvent yourself, do something for you that you would never have dreamed of doing before, go to a place on your own and explore it. When I split, I booked a flight to Paris, booked myself into a lovely central hotel and thoroughly enjoyed my 3 days mooching around the sites and markets. It was very liberating and made me realise that I didn't need a man to make things happen.
My friend who I guess is a similar age to you from your children's ages and username, has just been through the same thing. She is 6 months on so a bit further but has just joined the local health centre and goes swimming twice a week, a salsa class which she absolutely loves (and is full of blokes too!) an has enrolled at college to do an evening course.
She has also jus started to date again and joined an online site and has been out with two blokes since the split, one of which was a widowed man who she said is lovely but no spark and the other who is divorced and has some potential.
Life is all about what you make it, shit happens in life and it's how you deal with it that defines your future. Don't waste too many more days crying over your ex, he isn't worth it. Start your new life now and take small steps to make it better! Show him that you don't need him and that you have a better life without him! There is no better feeling than that believe me!