Congratulations! 
Would love to be the bearer of happy news, but all my pregnancies (on third now) have been equally horrible. As you describe, evil sickness, headaches, exhaustion, also had SPD. Whole 9 months there was something bad going on. Sickness lasted 6-16 weeks with 1st, about 4-16 weeks with second, and this one was 3 weeks until 20 weeks which almost killed me.
I'm in the headache/migraine stage, SPD getting steadily worse stage.
I also showed at 6 weeks!
I hope that you don't get it as bad this time, and hope others can come on here with more reassuring comments, but my previous midwife told me sickness gets worse with each one, and she was right (in my case).
Only tips I can give are keep eating, even through the sickness. It's really hard, but sickness on an empty stomach is worse. Try sickness bands. Try peppermint or ginger (you'll be one or the other). Try high dose of B6 (this helped with my nausea). Eat porridge before you go to bed, eat before you get up; keep that tummy with food in at all times. Drink plenty of water to keep you hydrated.
Last baby for me, I can't do this again!!
Good luck!! x