Hello! My native tongue is English, but I was raised in Mexico. Both of my parents spoke English at home while growing up, and I became fluent in Spanish just from being in Mexico and growing up with it. I married and remain in Mexico. My husband does not speak English, and I never got into the habit of speaking only English to my children. Now that they are older, I feel terrible because I realize that the only exposure to my mother tongue must come from me! My son who is 10 can understand pretty much anything I say to him in English, but he does not speak it, nor does he read it.
My daughter however who is 7 refuses to even listen to English. I have tried to get her to watch cartoons in English but she refuses to watch if they are not in Spanish. When I try to speak to her in English, she gt upset and says she does not understand. I am afraid that I am too late? How can I teach them when I am their sole source?
Any suggestions or advice are greatly appreciated!

First off, they are your children and they love you. So, just talk to them in English and they'll get to the habit of communication to your using that language. You can explain the benefits of being able to speak English.