Not Strong

Well, I bought a pack of smokes last night-no excuse-30 days into it and I messed up. What made my stress level so high that I thought this was the only way out or is it the fact the habit just raised its ugly head to show it still had a hold on me.
I do not know I am ashamed of myself, I made it 30 days, and failed. I bought 1 pack, and I am leaving in the car trying to limit my self to 4 a day.
I went walking yesterday with a friend, did not even think about smoking, was sitting here watching tv reading the news on the computer and it hit me. I got up went to the store to purchase the cigarettes.
I wish a former smoker would advise me of what I can do!