Thick hair help!! how to thin it?

Ok i wanted to ask this for a long time - I have really thick, bushy, poofy hair that doesn't want to straighten or curl (buts its naturally wavy) I've being growin it long and now that it's finally past my shoulders i've really noticed how crazy and thick it is!! I asked my friends about getting hair thinned with a razor but then my mum said she got it done once and it destroyed her hair! So I'm veering more towards layers but I dont want to lose any of my hard-earned length (ie. I haven't cut my hair in 2 and a half months and its grown about an inch which is a LOT for me and for someone whos always wanted long hair) would it affect the length much? And I use products for thick unmanagable hair and they work but their very heavy and make my hair incredibly greasy! Thanks for any answers! (^_^) luv ya! Xxx