Today is most likely your worst day. Day 3 is notoriously hard.
The best advice I read regarding week 1 on IP is to treat yourself as if you are sick . . . meaning take it as easy as you can, keep your expectations low as far as physical accomplishments (like running!). Just get yourself through week 1 and you will be feeling great.
During my week 1, I had a bad headache for days 2 - 4 and also had bouts of feeling very hot and sweating, rapid heartbeat, etc. All that settled down after week 1.
Hang in there.
NO GRIT, NO PEARL.