Need advice

So my boyfriend and I have been trying to conceive for probably about 4 months now. I always keep track of my period so I usually know 98% of the time when I'm fertile and ovulating. But even that isn't helping, every month we get disappointing news when af arrives.. I'm starting to think after having my two children that I can't even have kids anymore.. which is upsetting because I wanted a total of 3... I don't really know what to do, and him and I both are pretty upset about it. He wants nothing more he's always talking about it and picking out names and so on.. etc.. I haven't been on b.c for about a year or a little more. And I'm starting to wonder why I just can't seem to conceive .. please help .