How can you not feel guilty if you are close to or love this person ?
But how much pain can you endure ?
For me, A toxic relationship means it's going bad BOTH ways. You are hurting her , she is hurting you , and then a vicious cycle of hurt ensues .

Like you said , you knew it shouldn't have been to begin with so it was probably your guilt right there that kept it going , probably not wanting to hurt the other person.
Some things only experience can teach. And that's a shame.

I hope this relationship is not a marriage with kids. Ending that is very painful , ( for most people anyway ,the ones who have some feelings and might actually care. )

best to you