Pregnancy and motherhood may be many things (rich, rewarding, overwhelming) but the idea that they are always going to be "beautiful times" is an unrealistic idea that seems embedded in our society and probably causes a fair amount of grief.
I agree that you should probably see a psychiatrist for an assessment. Taking good care of your mental health is something that is worth the effort, even if it might seem a bit indulgent at times.
It is very hard when you have young children and you feel the way you do, it would be easier if you could have a few hours to yourself to relax but that's not possible so you are bound to feel pretty down at the moment, but to be carrying another child aswell? You deserve a good pampering!
I understand your concerns about bipolar, I am bipolar myself. You should see a doctor, I think you would feel better just to get it off your chest, as you are no doubt worrying how you will be when the baby arrives, you need to hear things will be ok.
Your hormones could be what is making all this feel so extreme, I have lots of episodes when I am hormonal, it can be a big problem.
I remember how hard it is when kids are little, they really do test your patience, but combined with hormones aswell!
I hope you can get some help and support,
see the doctor and then you could be feeling much better by the time baby is here..
Hugs
Forgot to ask if there were ways for you to minimise your stress and anxiety. Maybe some time out would help.
Can you see your doctor prior to Tuesday? Having a evaluation by a psychiatrist could also be a good idea.
I expect fatigue from the pregnancy and looking after your younger children isn't helping. Have you spoken to your ?husband and asked for more support?
I'm sorry you're feeling this bad. If you don't think that you can cope with things then there is always the option of going to the hospital and talking to someone there.