My car is shaking like shakira in reverse and 3rd gear ): please help for I am a fool
 

Hellos!

My car, debbie, is a 2006 Mazda 3 and she is being awful. when she goes into reverse or 3rd gear, she freaks out. Its like a shuddering, and shaking, and sounds like a thudding. The reverse shakes started last week, and the 3rd gear started this afternoon.

I absolutly am not a car person, and am just trying to work out how much she will cost me to get fixed/what's wrong with her. Any advice or suggestions would be great, I have no idea what on gods green earth is wrong.

♡ thank y'all