How are you supposed to exercise during the first trimester

I'm 9 weeks and feel like death. Everyone is talking about the importance of gentle exercise during pregnancy, but I'm at a point where getting across the room feels like a major achievement, and a gentle walk leads to a full day of vomiting and vertigo.

How do you do it? How is it possible? I'm at a loss and I feel so guilty that I'm not being healthy for my baby.