Hi there, I've been in your current situation twice now - the only main difference being second time I was a SAHM (which I personally found more 'draining' during the day than work)
The first time round my DD1 was very vocal and seemed happy about the impending arrival but she became very clingy (at night mainly but also during the day). DD1 was the same age when I was pregnant as DD2 is now and I pretty much had to deal with them in a similar way albiet 'tweaked' for their own personalities.
I'm currently 18 weeks pg and DD1 is 3.5, DD2 19 months. Both have always been pretty clingy but DD2 is amazingly OTT about it all, and yes, it is bloody hard isn't it !
DD2 wants me to lay on the floor of her bedroom and watch her sleep and for the past few weeks she screamed all night unless I did that. At the end of my tether, I asked my HV for advice and they told me to act as I would if I wasn't pregnant (i.e. don't cave in). It has been a very hard few weeks - esp since DH has been away at times (in fact, not sure if that maybe helped as I didn't have him to worry about too) but DD2 is now settling back to sleep if she wakes in the night.
We had to do controlled crying to get things this way and it got worse before it got better. This has all had the weirdest outcome for us - even though she is very clingy to me, she will only settle back to sleep if her dad goes to her in the night, if I go in she will leap up and scream as soon as she realises I am leaving. Neither of my DDs have ever been like that with their Dad - I've had to do everything up until now and it is a wonderful break for both DH and me.
I'm not going to say it was the easiest thing - I found my pregnancy hormones made it extra hard for me to ignore a crying baby - but it was worth it as I now get a decent nights sleep and DD2 has learnt that mummy sleeps in her bed not on the floor as requested. Can you ask you HV or MW for some support and/or advice?
btw it took about 3 weeks to get to an acceptable level of waking (lost track of time tbh), she still wakes up but settles very quickly thankfully.
hth - good luck