Just put "resigned to seek other career options" on your resume, and don't use them as a reference. I've had LOTS of jobs end that way, that I've always gotten around it that way. I don't normally get questioned from that. I've had to lie about past jobs to get new ones several times, but it's always worked out. It sounds like it was a bad situation, not something that was your fault. I've dealt with several toxic jobs myself, and I reacted exactly the same way. Try not to blame yourself too harshly.