Sleeping location

My daughter will be 16 weeks this week and I had her sleeping in the rock n play up until a few days ago.  (She's gotten quite tall quickly). Been trying to get her use sleeping flat but it's hard.  Due to space issues in my home (2 bedroom condo with her being our 3rd child in it - her brothers share a room).  We use the pack n play in our room but she seems to prefer sleeping next to me after nursing. How can I get her use to laying flat?  She slept amazingly in the rock n play but is a super tall baby now and it seemed really uncomfortable to her after a bit.