Funny that I read your post just as I was logging on to share a good news story. But I'm sorry this has happened to you too 
Just occurred to me that I used to frequent this bit of mumsnet a lot (and I mean a lot) but I haven't been here for a while. We have a DD aged 6 and when she was 2 we started TTC a sibling. A long and sorry tale followed of 4 MCs, 2 of which were MMCs and resulted in having the ERPC operation. Tests showed absolutely nothing wrong with either of us - and we've been looked at by two different hospitals, (St Mary's in Manchester being fab).
We were awaiting the final results of a genetic test to see if the 4th MC was a molar pregnancy, which might lead to cancer for the mother (me, eek!) and told not to TTC in the meantime. I was distracting myself by trying to lose weight and actually take some exercise rather than console myself with cake like I usually do. And I lost half a stone. Then it occurred to me that I didn't know when my last period was... POAS and sure enough, 5 weeks pregnant
. Just as we were adjusting to this, I started bleeding.
So we went in for our results, panicked, convinced that (a) I had cancer and (b) I'd lost a 5th baby. But, BUT! No! It hadn't been a molar pregnancy so I was fine, and a scan detected a heartbeat so all was ok! Really really tense few months with lots of bleeding for no apparent reason, but I carried her to term and now she's 9 months old and currently chewing the face off her teddy 
It now feels like all the horribleness of those 4 years just never happened. But I know how it feels. And no one can tell you when the right time to stop/carry on trying is. I felt like I had no choice but to continue and my DH was in total agreement. But I'm not sure I would have kept my sanity without the specialist midwives at St Mary's.
I used to like reading about happy endings when I was feeling low, so I do hope this helps you or anyone else in the same boat. Really don't want to sound like I'm bragging. It's all just the luck of the draw isn't it. Life can be a bastard sometimes.