Hi,
I just wanted to say that i think many parents experience this type of anxiety (although it may manifest itself in different forms, everyone is different), my mom does as well, so you are not alone. The bottom line-- is this anxiety having negative effects on your daily life and basic functioning? This is a fundamental guideline for most types of disorders. Are you having actual symptoms of distress, or is it just more of a preoccupation? (IE panic attacks, or physical reactions, illness etc). If so, then you probably need to have some professional counsel.
Remember, your children growing up can be a great thing. If they are able to be independent, educated and successful on their own it means you have raised competent human beings. That's awesome! CW and Caroline are right, once your kids are self-sufficient you will have more time to focus on yourself. Try to develop more relationships with friends and family, even though it may be tough, it will help to keep you socially healthy and provide you support.