Help!! What's wrong with my car?

I have a 2006 Honda Civic Si. Yesterday morning I drove my car to the store. When I got back in.. started it up.. and tried to put it in reverse it grinded.. tried 1st gear.. grinded.. turned off car.. turned back on.. car shifted just fine.. What could this be? Clutch? Tranny? Or Master Cylinder? 3rd is sometimes hard to get it into.. doesn't grind.. just feels like it doesn't want to go into 3rd..I have a friend that has the same car.. he said it was most definitely master cylinder especially because of the difficulty getting it into 3rd gear.. just looking for comments and opinions to see what I should do from here.. Thank you in advance!!!!

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