Ttc #3 for me but #1 for dh

I have a 14 yr old son and an 7 yr old son from previous marriages. Dh and I want a little girl together. We have been not not trying for six years. We have been actively trying for the past three months with no luck. I turned 40 Christmas Day so time is of the essence.  Any help you can provide would be appreciated.  Ds#1 was difficult but ds#2 was a surprise.