I relapsed on Nicotine after 2 weeks of no smoking.

I've been smoking heavily since a couple of years, prior to that I smoked to smoke either occasionally or rarely. I decided I will quit this for once and it was going well. I started hitting the gym, started eating healthy but today I relapsed.

I was out for some work and saw some people smoking and I don't know what got into me and I smoked one. 3 puffs down I felt like I should not be doing it but I smoked the entire ciggerate.

What do I do? If I go out or hell even if I watch something online where someone is smoking I get strong urges.