Been broken up with a girl for a couple of years now. Seeing someone else.

Finally dropped off the last of her stuff. Left it in the mail and left a message on her phone.

Her response was just "thanks"
Why am I trying to read into it. It's really bugging me. How do I let it go

What are you reading anything into this at all? You waited a year, and finally handled things so of course you have to deal with all the old feelings being stirred up again. I mean what's to read into a thanks for my stuff back message?

Let it go, and get back to what you are doing NOW in your life. The feelings will fade if you let them.

Thanks means thank you.
Nothing else.

It's over; it has been over for a long time.

Time to accept that.

That 'thanks' is nothing more than a polite formality. So do not read anything more into it.