Aiyoh that sounds really tough lah, most mummies morning sickness gets better after first trimester but some mummies kena the nausea all the way until second trimester or even whole pregnancy! It's not super common but it's also not unheard of, so you are not alone in this, some unlucky ones like me also had nausea drag on quite long. But since you are already 18 weeks and still vomiting every night until cannot keep food down, please inform your gynae at your next appointment or call them sooner because they can prescribe anti-nausea medication to help you, no need to suffer in silence and risk not getting enough nutrients for yourself and baby!
This one better see a doctor soon, even though some nausea can linger into the second trimester for some mums. Vomiting almost every night and trouble keeping food down needs checking out because your body needs the nutrients, and the doctor can rule out anything else and help you feel better. Don't wait on this one, please make an appointment with your gynae or go to A&E if you're feeling really unwell.