How do you leave a relationship

Mr

I've been wanting to leave my marriage for awhile now, my husband has been cheating on me with his ex wife and he's extremely mean, constantly name calls for no reason. But when I start showing signs of wanting to leave he switches up and starts acting nice and when I tell him I know what he's doing he explodes, name calls, tells me if I leave I ruin the family, etc. Excuse after excuse for his mean and unloveble behavior

I just can't do this anymore. I CANNOT. I know in my heart I'll never be happy married to him, never. He's far too aggressive and unloving and his obvious cheating with his ex wife. but it's extremely hard to leave when you're married, have kids, etc. There's so much change I just don't know where to start or how