it does take practice to manage to free motion quilt on your domestic machine- it is best to practice, practice, practice- when you are happy with the quilting on a small (like 20" square) piece then try a 36" piece- then 48" ect- work your way up until you are quilting large quilts- you might want to check around & see if there is a beginner quilting class in your area- when i first took one at the lqs we made twin sized turning 20's - the class really helped learning tips/techniques for manuvering the bulk (mine was a flannel one-both sides)
after quiting 2 or 3 twins i went to a full- then a queen- i am presently quilting (on the domestic machine) a whole-cloth queen- i have a friend who does king sized quilts often & quilts them all on her domestic- it just takes practice & patience to get there.