There are many ways to tell when it's time to end a relationship, D/s or otherwise...
If you keep telling yourself 'it will be better if.......'
If someone tells you they are wrong for you, and you can't come up with why they are right for you
If there are things you just can't get past
But most of all, if it feels wrong to you, I guarantee you it is...nobody knows someone better than themselves, so if you're looking for reasons to get out of the relationship, you already know that it's time to leave it, you just have to do it.

As sad as it is, there is always light at the end of the tunnel, good luck!