I messed up today.

I am 32 and was a 15 year smoker and quit about 3 weeks ago. Today I found an old pack with a cigarette left and… I just couldn’t resist. I didn’t realize how fucking stressful it’s been until I took a couple drags and my whole body relaxed and the rush hit. HOW do I keep from going back to smoking? How do I get that same relief nicotine brings? It feels like ill never be able to think clearly without it. HELP!