You just said that you have three great kids. That is something to be very proud of.
As for relationships, that takes two. You are not the only person who needs to take responsibility for those failures. Maybe therapy can help work out your relationship issues.
The first thing you need to do is give yourself credit for your accomplishments. Be proud of yourself and then you will not settle for men who take advantage of you. Once you learn to love yourself, you are able to allow others to love you in the way you should be loved, in a healthy way.
Being on your own at first may seem hard, but you will learn to enjoy its moments. You do not need a man to complete you. You can be complete on your own. A man can compliment you, the same as you can compliment a man, but to look for completion in another human only leads to disappointment because we cannot control anyone else.
Find some good friends that you can lean on. Take classes. Take up a hobby. Keep yourself busy in things you love to do. Find out who you are and go from there. You might just find that you really enjoy yourself and the company you begin to keep.
I wish you luck on your new journey.