Oh LoveYouForever, you poppet. Dry your eyes, have a 
, a 
and a big hug with your little one.
I've only just seen your post (been on yet another driving lesson) and I'm really, really, sorry you're feeling so upset.
I did wonder if your return to work was preying on your mind as you mentioned it in passing a while back. I can't really offer much advice I'm afraid as I couldn't face going back after DD arrived. Whilst - on balance - it was probably the right thing for our family, I do often wonder what life would have been like if I'd had the courage to grasp the nettle at the time.
Although I feel very lucky to have had the option to stay at home, the stark truth is that I have permanently wiped out damaged my career prospects. We have also taken a huge hit in terms of income and, even now, I find it uncomfortable being entirely financially dependant upon DP.
Is there anyway of separating your feelings about returning to work out from your feelings about ttc? Could you negotiate less hours? Are there any friends that have gone back recently that you can talk things over with? Are you happy with the childcare you've got lined up?
Obviously I don't know about your financial situation, but would it be possible to give yourself another 'exit route' that isn't dependant upon falling pregnant quickly? For example, could you agree with your DH that if, after 6 months, your return isn't working out, you will resign? Or look for a part-time role somewhere else?
These may all be silly suggestions but I just wonder if there is a way of taking some of the pressure to conceive instantly off your shoulders.
Can you focus on some of the benefits of going back? A proper lunch break, more adult company, more money coming in, entitlement to maternity pay when DC2 does come along etc..
Even if you aren't pregnant before you go back, it is likely you will be before too long and having your DC1 used to spending some time away from you could be a huge bonus when you are trying to juggle a toddler and a baby. For example, you may be able to keep your DC1 in a few hours of childcare each week so that you can spend time snuggling and bonding with your teeny one.
I'm probably just rambling now, sorry for the epic post.