It is commendable that you try to observe the Five Precepts. This is not an easy thing to do for modern people.
The Five Precepts forbids sex outside of marriage. This does not apply to the general believers who have not received the Five Precepts.
Unfortunately, your case is quite common because most temples do not provide the proper training about the Five Precepts before transmitting them. We train our disciples to make sure that they thoroughly understand the Five Precepts Dharma before transmitting it. In that way, they can avoid problems like yours.
You need to perform a special repentance Dharma with people who can be of real assistance. In Northern California, you may contact Ven. Heng Sure at the Berkeley Institute of World Religions to see if he is available to help you. Otherwise, you have to come down to Los Angeles and see me.