kellijh have you been to a Dr yet about your cycles and not been pregnant after 18months ttc? Perhaps now is the time to seek their opinion or within the next 6months if your feeling ready for that. Its very frustrating isn't it? I also have been ttc for 18months and have seen numerous Dr's and Gynos and I know why I am not pregnant because of this and finally have a plan of action if I am not pregnant within the next 6months.
To start with I had tests to confirm I was ovulating which I was and a scan to check for cysts and polycystic ovaries. Then I had a laparoscopy (for ectopic pregnancy not for fertility investigations) and it was then that I was diagnosed with what our problem is. Although it sucks and I am grieving becuase of why I can't fall pregnant it is good knowing why (not that I am saying there must be a problem) but it is helpful to know if everything is ok or not.
Best of luck I hope it happens for you soon or you get some answers to help make you pregnant.