Get mouse position is providing the position of your mouse on the screen in 2D space. If you want something to happen in your level (in 3D space) you will naturally get weird results (I just fairly recently learned this the hard way just like you :D).
Anyway the solution is to “get hit location under cursor”
You will get a usual hit result which you can break to get the hit location which is what you want to use.
This also enables you to ignore actors like other pawns or dynamic level objects or to check for everything just as you please.
I hope this helps.
Cheers