Hey EmmyJo, I am in a very similar situation to you - 31 weeks, work 4 days a week, also with toddler who has suddenly started to tantrum etc. I also have SPD which means I am finding it hard to walk far. I feel very guilty on the days I have DD and I'm not able to walk to the park with her etc. I am also physically very tired and not sleeping now - so all in all, feeling pretty rubbish too!
I am hanging onto the hope that having newborn will be easier - at least physically. Saw a friend today who has 2 DC and said you forget how much newborns actually sleep. I am planning to keep DD in nursery a couple of sessions a week when my mat leave starts so I can rest and for after the baby comes.
In terms of the tanrtumming, I think it is probably your DS' age - I've read this is the phase they get more, erm, 'challenging'. Although I do think my DD has started feeling a bit threatened by the bump.
I have been looking into toddler and baby groups in my area so I can start to plan things we might do when the baby comes. There are more than I realised which is reassuring.
Anyway, just wanted to let you know you're not alone. Be kind to yourself, and make sure your DH is supporting you and picking up some slack too (could discuss in counselling maybe?.