Been there! 18 months and 2 miscarriages. I would HIGHLY recommend the book, "Taking Charge of Your Fertility". You will be able to tell EXACTLY what your body is doing, if you are ovulating and when, and when you are most fertile.
I too am of "advanced maternal age". I'm 39, and had my last 2 kids since I was 35. I would also HIGHLY recommend the book "What Your Doctor May Not Tell You About Premenopause: Managing your hormones between 30 and 50" by Dr. John Lee. Tons of interesting and useful information. I found out with my second miscarriage that I was low on progesterone, so I started myself on a progesterone cream and it was during my 2nd cycle on the cream that I conceived, and am now at 26 weeks. Be cautious of what your doc may advise, only because they simply cannot keep up on all the research and it is most important for you to know your own body and how it works, and to follow your instincts. Secondary infertility is actually more common than primary and with secondary infertility, 6 months is plenty long to look into what's going on. None of this 1-2 years of trying...you obviously can conceive. I pray it works out for you...
Oh, and just a note of caution, when you DO conceive, it will be the most mental pregnancy you could imagine! You will analize EVERYTHING!! :-)