The tears are emotion, but since you are pregnant, you need to make sure that the racing heart isn't a medical condition.
Example: Mitral valve prolapse is benign and normally not symptomatic, but during pregnancy it can cause symptoms of anxiety.
Meanwhile, I found that if I took myself off somewhere and had a really good cry, afterwards I could pull myself together for a good stretch of time and feel calm. It's when I kept trying to suck it up that I had problems.
Also, if you lie down after a hard cry, elevate your head so your face doesn't get all puffy.
Drink LOTS of water.
Do 1 unexpected, nice thing for your 5 yr old every day so he feels loved. Even if it's just a hug/kiss/Iloveyou.
Everybody, soon or late, sits down to a banquet of consequences.
~Robert Louis Stevenson
Its so hard to go through this when you feel like yourself, but the pregnancy stress on top is just the rotten cherry at the top.
Im sending you hugs.
Talk to your doctor about the panic attacks. You firstly need to make sure there isn't a medical reason for your symptoms. What you are experiencing is probably a result of the extreme stress you are under. Tears which are so close to the surface and easily start flowing, for me, have been about feeling overwhelmed and deeply hurt, and I found I had to let them out. Try and control your breathing and slow it down when you feel you can't breath.
Keep talking to those who are your support. Accept any practical help they are offering and let them know how you are feeling. It helps to talk to your most trusted family or friends. Look after yourself, maybe with relaxing warm baths before bed.
There's always light at the end of the tunnel, just pray it's not a train.
"When you understand the nature of a thing, you know what its capable of"...musashi...the book of five rings
Some wonderful advice here, as always.
I wish you and your little ones a brighter future.
My thoughts to you x