What you should be doing is rendering each part as you go along. You will be surprised how fast a song comes out of many takes.
A good way to practice commiting to a take is just to smash everything together in a 2 track stereo recording via 8 16 or 32 bar takes. Use an inline recorder(s) like Tapeit and just keep recording more and more chunks of the song.. The more you do this the more you'l percive the length verses the arrangment plus automation etc.
Remember this, human senses become dulled very easily. Just like when you listen to a mix too long your ears start to fool you. Well its the same thing with listening to a loop. Basically you brain becomes bored and tried to protect you by telling you that the song sounds bad and you should delete it. For instance if you ever go back to a song a couple of days later it usually sounds good (or bad depending
I basically solved this problem for myself by recording many takes and then just combining them up. Eventually you'l get so good that arranging will seem like 2nd nature..

