First, realize that you're not alone. In fact, secondary infertility issues -- where a couple already has one or more children, but is having trouble getting pregnant again -- are just as common as first-time fertility problems. It might also help to remember that the majority of people who seek treatment for fertility problems do go on to get pregnant eventually.
If you start to feel overwhelmed by your feelings, seek out some emotional support. It's perfectly natural to be just as stressed, frustrated, and depressed as someone who's dealing with infertility issues while trying for a first child. In the past, couples facing secondary infertility were extremely underserved when it came to counseling and support groups, but now organizations like RESOLVE (The National Infertility Association) are adding more services for couples with secondary infertility problems.
Couples struggling with secondary infertility also face the unique issue of dealing with their problems while raising a young child. It's natural to want to protect your kid from your sadness and frustration, but what do you say if he or she asks about having a sibling? If your child is old enough to understand (starting at around age 4) then it's okay to tell him the truth in a manner he can understand. If your child happens to see you crying, don't try to hide your emotions from him (an overactive imagination can conjure up all kinds of terrible scenarios). Instead, say that you've been trying to have another baby, but it hasn't happened yet and you're a little sad about it. As a rule, when talking to young children about sensitive subjects like this, it's best to not give too much information, but to just answer their questions in a truthful, yet hopeful, way.