There are so many toxic people out there. It's almost impossible to not date at least one toxic person in their lifetime. However, you never hear of someone saying that they are toxic themselves. It's possible that YOU may be a toxic person in a relationship.
You make your partner ask you for permission before they go out or do something.
If your significant other wants to go out with their friends, they shouldn't have to ask you, "Hey X wants to hang out tonight is that ok?" Part of being in a relationship is being with someone you love, but also still being able to be free and be yourself. You aren't their parent. You shouldn't tell them they can't or not to do something.
You are never the one to apologize.
Granted, I totally understand that it can be hard to admit that you're in the wrong sometimes. Personally, it takes me a bit to come around and apologize, but I can't say I never apologize. For example, if I'm on the phone with my boyfriend or even in person and I'm being overdramamtic or being a bit mean towards him, I always notice it eventually and I always apologize for it. I never want to make my boyfriend feel bad or that he deserves to be treated badly. However, if you never apologize or think nothing is ever you're fault...sorry but that's toxic. You have to be able to admit when you're wrong or come across the wrong way to someone.
You aren't loyal to them.
Not onlyt does this include cheating, but this also is a respect thing. If you are blowing your S.O off to hang out with friends even though you and your S.O had plans prior, that's pretty low, nor is that loyal to your S.O. Cheating, in my opinion, is one of the lowest things you could do to someone. Not only does it break their heart, but it makes them question everything about themselves. "Am I good enough?" "What's wrong with me?" "Am I not pretty enough?" Instead of cheating on someone and hurting them 10x worse, just break up with them. Give them time to be alone and get over you before you go and cheat on them.