Feeling discouraged..

I've been using this app for a few months now. I've religiously logged me days (period start & stop, when we have sex, everything) each month when the app says that I am ovulating we have sex. Each month I get excited thinking this time we did it, this time we had sex on my peak day. But the next month I start my period. We have a son that will be 3 in November and we are wanting another one. I've been feeling really discouraged each time I take a pregnancy test and it comes out negative.