Toxic relationship

How do I get out of a toxic relationship. I’m 17, turning 18 in four months and I’ve been stuck in a toxic relationship since I was 14. It’s been four years and I know that I deserve better but I just can’t seem to let go. Maybe because it feels like I’ll be alone if I do? Or the possibility that I won’t find anyone else ? All I know is that it isn’t good for me but I’ve tried so Many times to walk away and it never works . I walked away once 2 years ago for about 6 months but somehow we ended up back talking again it’s like a never ending cycle .I can’t escape it and I don’t know what else to do.