You're not married!
If you want to date someone else and are not in an "exclusive relationship status" you can date both people. On the other hand if you are in an "exclusive relationship" be honest with yourself.
If you were really "into your mate" you wouldn't be contemplating what it would be like to be with someone else.
Know yourself, Love yourself, Trust yourself
Make sure you're not one of those people who treat relationships like jobs. They will only leave a relationship when they have another one lined up. They're scared of being "free".
If your relationship has run it's course in (your mind) then leave.
Don't waste you or (your mate's time) simply "going through the motions". You are responsible for your own happiness!
Allow your mate to find someone who truly only wants them.