Maybe its because you feel so miserable people take it as being intimidating, i know that is one of many of my problems!!
I know its hard with small children my husband isnt my daughters father she was nearly 3 when we met, but you can use that to your advantage, take your children to the park, and to clubs and all sorts you will meet many single dads in the park on saturday/ sunday afternoon, take a picnic and pretend if you have to that you have everything you want, people cant get to know you if you dont like yourself, try to find the things you enjoy doing and see if you can bring your children into it. ( I loved rollerblading and so i bought my daughter a pair and then pestered my dh until i got a pair so i could teach her!
)Your not alone there is always someone on this board who wants to relate to someone else, and i'm sure your chldren will enjoy going to the park or bike riding with mummy, who knows you could crash straight into the man of your dreams.
Just remember that being in a relationship isn't the be all and end all, if you have other problems they will be there whether you are single or not...
Hope you find something to make yourself happy