It's hard for empaths to sort out their feelings, since they feel just like ours...
Grounding is an essential Empath skill but it might not help here since the foreign feeling IS there. You are feeling it. So when you're grounding, you're just paying even more attention to that feeling inside you.
A quick way to sort this out that I use all the time is Turning Down the Volume. It's the first technique in the Empath Survival Program. By turning down what you pick up from other people (do NOT try to turn it off), what's left behind is yours.
Ironically, it solves two problems: 1) Identifying feelings that are not yours 2) Preventing external feelings from affecting you.
I hope this helps!