Raising a child by yourself can be done well, but there are some common challenges:
Less money - 1 income is less than 2.
Kids may feel insecure, like something key is missing from their lives.
Insecurity leads to self-destructive behavior, including risk-taking, lower grades, substance abuse, and even personality disorders.
If you are raising a child by yourself, your workload has doubled. It's just math.
Consider the following cycle:
Double the workload means more stress.
More stress leads to burnout.
Burnout leads to worse parenting.
Worse parenting leads to worse behavior
Worse behavior leads to more stress.
Rinse and repeat, only increasing each time.
However, it's not automatically hopeless. If fact, single parents actually have the opportunity give more one-on-one attention to their kids, because it's just them.
Summary:
A great single parent beats 2 bad parents any day. However, having both is usually better. Kids without a dad may need male energy, and kids without moms may need a woman's touch. In these situations, try to enlist the help and time of a friend, relative, nanny, or someone who can supplement the missing gender influence.