Welcome and congrats on 18 days.
The good news is the worst is nearly over. The first 3 weeks are the worst. You have done most of the heavy lifting.
Stay close to the board and post an SOS if you need encouragement.
Read widely the posts on this site to raise your awareness on how to continue your quit. it can last a while bear with it.
The brain fog thing is a sign of improved oxygen supply to your brain it can be annoying but its a sign of improvement in your circulation . Consult a DR if you are concerned about it.