Glutton, just to say hang in there, pregnancy and small children are a really tough mix. I really, really struggled with pregnant with ds2 at times, as ds1 was very tantrummy and I was also childminding. I was utterly exhausted.
Once Id had the baby, it was SO much easier, I was more comfortable, I actually got more sleep, luckily the baby was good so I was able to spend time with ds1 too.
Well done for being calmer with the kids, you've taken the first step, you're aware of it, have acknowledged it and are doing something positive, wihchis really brilliant. Its great the the kids responded so well, which just proves that if you keep it up most of the time, you can turn the situation round.
My suggestion is, if you have the energy, get the kids out to the park while the weather's good. Take a picnic blanket, snacks, balls or small toys/books. Find a big area and sit down in it and let the kids run free. Not a play park, as you'll end up running around after them/intervening in squabbles. If you are out, you will feel calmer and you wont be able to shout at them! If you find a nice big open space, you wont have to interract with other mums at the park (i sometimes find that stressful in itself when the kids are being a bit challenging). Or, if you are physically up to it, put one in the buggy and one on his bike and go for walks to wear them out, then get them in bed early in the evenings.
Make sure you agree with dh certain days and times where you get time off, when you can hand him the reins and take yourself off for a coffee/lay in bed to read your book/sleep/do something else undisturbed. Agree it and stick to it, then however monstrous the day is, you will know that in X ammount of time you will have some time off. Let him do the bedtime routine on certain days and you stay downstairs and relax. Put on your ipod so you can hear chaos and aren't tempted to intervene!!
I really, really think that you will feel better once the baby is born, talk to your midwife and tell her exactly how you feel about the birth, she might be helpful and able to calm you down. Do you have people that can help out once the baby is born?
Good luck, and dont be hard on yourself, kids are bloody hard work sometimes, and we all have less finer periods of parenting that we'd rather forget about. This WILL pass, you will be fine xxx