Disappointed My Boyfriend

I had been smoking cigarettes off and on for about 10 years. I quit a year and a half ago. I started up again last month when I found out my 30 year old brother died.

I was asked by my boyfriend if I started smoking again. I knew he knew because he could smell it in my hair. I said yes and that I am sorry and that I threw them away.

Why did I do this? I didn't want to start smoking again. It is the # 1 turn off to him. I wasn't smoking when we started dating and I told him I wouldn't start again. I didn't mean to lie/hide it.
I feel so horrible. I don't want to lose his trust. I'm not sure what to say or do.
smoking is temporary. your bf should be understanding under the circumstances.