Sorry to inform you but it is the fact that you are a single mother.
I will give you some insight into the male brain.
When we see women, we see an opportunity for sex. Each women has its own challenge and some are easy to get in bed, while others are difficult. To men, an attractive, rich, confident woman is very hard to get in bed because we think she can have any man she wants.
On the other hand, unattractive, low confidence, poor women are easy to get - especially if they become dependent on what the guy has like money or if they are slick with words and feed compliments to the girl to make her confidence high.
Many men scan a woman for these insecurities, whether they are real or perceived. For yourself, you are a single mother. The knee-jerk reaction of a man is that you're poor and need to find someone to help you pull your life together. That's not to say this is true in your case, but it's the image that men will form in their brain. When they think this, they will see you as more willing or even desperate to try and attract a guy to stick around. Men won't need to try very hard with single mothers, because they know that the woman has a lot invested in the relationship and is trying to make things work to improve her and her kid's life. Unfortunately, the guy isn't actually around for you. He is around for free sex because the woman is often willing to give it. He will also have little interest in taking care of a biologically unrelated child from another relationship.
My advice to people in your situation is to stop looking for now and focus on raising your child. You will have to weed through hundreds of guys looking only for sex, in order to find someone who's willing to stay with you for the long-run and help raise your kid. Frankly, this brings a very unhealthy living environment to your child, if a new man appears in your kids life every few months. Along the way, you will be repeatedly hurt, and this may have unintended psychological effects on the kid and their issues with trust.
However, what I've said is a very general blanket solution. Without knowing the specifics of your situation, I could very well be wrong.