I told my partner that we were in an open relationship now despite what she wanted
Why did you say that you guys are in an open relationship ?
My partner wasn't giving me what I wanted, therefore I'm out of this relationship now.
If you don't feel compatible with your partner then you should break up with her. She doesn't want to be in an open relationship so you shouldn't force her in one.
What if I suggest open relationship because I think it's more ethical than cheating?
There is nothing wrong with being in an open relationship but the decision has to be made mutually. If she doesn't want to be in one you should not force her. Just break up with her and find someone else.