Help!!!! Origin will not go to courser at center??

Im trying to mirror an object in Blender, the origin of the object is out of place so it will not mirror right. Ive been modeling this car all day and i was using mirror and it worked fine until now…

I set the 3D courser to center and then I set the origin to 3D Courser like I always do… but the origin will not go to 3D courser in for me to mirror the object this time!

before i do anything this is what the scene looks like:

After I set 3D Courser to Center and then set Origin to 3D courser this is what happens:

and each time I Ctrl+Alt+Shift+C and set origin to 3D courser the origin stays where its at and the object moves farther from the origin? why is this?

Here is what you do,first move the mesh in object mode to where you want the origin point,after that go into edit mode and select all the vertices (A) and move them to where you want them (the origin point will not change while in edit mode)

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