How do I tell my mom?

That my husband cheated on me? I only want her to know what actually happened. It’s going to be a big thing to ask her to keep it from my dad. Because he might actually kill him. I’m supposed to leave again in the 3rd and I’m afraid to tell her Because she might keep me here. I plan on packing all my shit one day while he’s at work and driving back home. Without telling him. I just need to know how to tell her. I’m so heartbroken