I think you're on the right path.

Let me restate what I think you're saying. While you're with her you feel you have your boundaries set ok, but afterwards you realize you didn't?

To me this means you are giving out more energy than you have; she's sucking your energy from you. That could be what you are "sensing" while you are with her, and then once you are alone you realize how drained you feel?

Set stronger boundaries. Find some key phrases you can say that allow you to show compassion but doesn't give her all your energy. Recognize key phrases or words SHE says that seem to trigger your empathy, then remember to say NO to yourself to stop that from happening.

Don't go back on your boundaries. Once you've identified them, and her triggering, promise yourself that even though you might feel fine at the time, you WILL hold to the boundary. I think you'll feel better afterwards.

Hope something here helps you.