‼️URGENT‼️ my partner said they’re thinking about cheating on me, what should i do?

to preface, this is not the first time they’ve said this. we have had a very toxic relationship in the past but recently we have made a huge amount of progress. we’ve been together for a year and six months. in the beginning, there was cheating back and forth. i have cheated twice, both over a year ago. i can’t even count how many times they’ve done it, all leading up to recently. we got into an argument today because i ran errands and stopped at one extra store when they wanted me to come home and spend time with them. (important note: we are long distance so the things we can do together are limited to facetime). then, they said “you cannot expect someone to not wander outside their relationship when they’re unsatisfied from within”. i immediately blew up. i know this relationship is extremely toxic, they are an extremely manipulative person, and i know i need to leave. i just have hope sometimes that it will work. they are abusive in all ways, including physically when we have visited eachother. i’m sorry for the long paragraph i just really need support. i’m only 19 and i don’t want to do this for the rest of my life but when i try to leave they blackmail me and i don’t know what to do.