Oh you have my sympathy!
I had horrible nausea in my first pregnancy and with this twin pregnancy as well. And it lasted until about 22 weeks. I am sooo glad that stage is behind me.
What worked for me is to eat frequently--even in the middle of the night. I kept a ziploc bag of nuts by my bed for the first two months and then switched to PBJ sandwiches. Also, protein seemed to help better than carbs. Before getting pregnant, I was mostly vegetarian but during my pregnancy I've always kept meat around. When I was so nauseous, I'd have DH cook it and then I'd just go to the fridge and grab a piece to snack on when I was feeling awful. Not only did it help with the nausea, but you need a lot of protein in a twin pregnancy. I too ate McDs and BK every once in a while. Those burgers always made me feel better
The other thing--don't be afraid to try prescription anti-nausea meds. I took Zofran with my first pregnancy and with this one too. It was the only thing that worked for me--I tried other meds, ginger, B6, accupressure, etc. etc. The Zofran helped me live my life rather than always wondering when I'd have to run to the bathroom to puke. All of the anti-nausea meds that OBs commonly prescribe are well-studied and very safe.
I'm now 36 weeks and my ultrasound yesterday estimated that the babies weigh 6 1/2 and 7 lbs each and my placentas look very healthy. I attribute that to eating a lot of calories, mostly in the form of protein. I've gained 60 pounds total, most of which I put on in the first 6-7 months (I think I had gained about 30 lbs by 24 weeks). Right now there's just too much in my belly to eat much
so I'm not really gaining anymore.
Good luck! Hopefully this stage will pass for you soon.