It must be so hard for you TTC for 20mths, I was getting really upset and depressed after only 8mths but finally after I relaxed more and told myself we were giving up as it was too stressing it happened, I finally fell pregnant with our 3rd.
Hubby and I actually went on a night away and totally did relax and enjoy our time away from the kids and that is when I fell smile
So I really do feel the key is to relax as the more you do the stress the more it does not happen. I actually did use an ovulation kit this cycle to fall and it worked well, have you ever tried one? I thought the kit was great as it did pinpoint the right time as my body cycle was all over the place as well. How long did it take you to fall with your DD?
Don't feel like you are a failure you are not, you have a daughter already so I'm sure it will happen again for you. Take care of yourself and I know its easier said then done but try to relax and take your mind of it all.