frustrated with my cycle! help!

Karen
So I have only been TTC for 4 months. Each month my period is either late or early. It's random so of course I don't think we are aiming for the target on the right days. We are having sex on the target days and whenever we want really....Just frustrated. Any advice? 
P.S. We both have a child from previous marriages so I know we can make them. Lol