Greensleeves,
I'm not sending you a dagger, just a hug. I don't know where you live

but you deserve a hug. So here's a cyberhug.
You sound like you need a break! Are you taking any time for yourself these days? Even something as simple as buying a yoga video ($10-20) and doing that 2x a week in the evening might help. Even if there is no time or funds for getting out of the house for yourself, see if there are simple things you can do for yourself once a day--a bath, lighting a scented candle, rubbing your feet with nice aromatherapy lotion.
It is so tough to be pregnant with a toddler. I'm pregnant too (20 weeks) and I felt that ds and I had a tough summer with my morning sickness and his tantrums. The thing is, I never regretted when I was overly kind to my son (otherwise labeled as being too permissive

) but I do regret the times I yelled at him and spoke too sternly.
Could you count to 10 before replying to dd (I try to do this when my son is having a tantrum or whining).
A book I read that really helped manage my irritability is When Anger Hurts Your Kids. A very practical, helpful book that discusses the developmental stages of children. Basically the premise is that kids have behaviors at every stage that can be deemed "unreasonable" "Bratty" "driving me crazy." The key is not to label the behavior as such but to be more observant and less judgmental. Such a helpful book.
Anyway, try to give yourself a break.
I just read the rest of the posts and wanted to suggest waiting until after the baby's born to take antidepressants? I thought about going on them myself during the pregnancy (depression is something I've dealt with in the past) but felt so horrible that I decided to stay med-free for the pregnancy. But it may be something worth revisiting (for me) once the baby's born.
I've heard that resuce remedy, chamomile tea, and 66 supplements can help but I've never done these things. Also the fatty acids (but I don't know if these are okay to take during pregnancy).
I do second the recommendation on exercise. I just took a 20 minute walk and felt so great afterwards. Exercise doesn't have to be a huge workout at a gym--a short walk can help. Or putting on some dance music and dancing around for 20 minutes with your daughter. Also, writing in a journal may help you too (works wonders for me

Your comments about your daughter telling you not to sing made me smile. My son does this and I have to remind myself that he is not "bossy" or "rude" but just two. Again, it's all about reframing our thoughts.
Try not to be so hard on yourself.