Life is ever evolving which is why you need to remain flexible to change. Although it can be difficult in the beginning, if you stick with decluttering and organizing often you'll find things will start to get easier and start to flow on their own. Things change as circumstances change.
If you add new things to a room often times it will change the function and flow of the space. Remember that any time your life changes, it's time to re-organize your home. Expect to re-organize and declutter your home after the birth of a baby, a job change, the loss of a loved one, or simple things like starting a new hobby.
When I was living in a 600 sq. ft. apartment I wanted to learn how to play the piano. So it was necessary for me to re-think the layout to fit a keyboard, bench and music. I looked over everything in the room, donated a few things, and shifted items around to accommodate my new hobby without overwhelming my husband and pets. If I had not taken this step, I would not have had the space to practice beautiful music.
When deciding what to donate focus on items which are no longer useful in your life. Could some items be helpful later? Maybe, but if you can replace them affordably when needed, then out they go. I find it's much more functional to remove than store.
You may not have a small house, but your need for creating a functionally organized space will still remain the same. If you don’t declutter and sort through your possessions after a life change, or a shopping spree, then it will only be a matter of time before your home becomes an un-functional, overwhelming space. Your home should always be your place of peace, not a place of pain.
Remember that an ounce of organizing NOW will save you from a pound of clutter busting later. It's time to get up and take some time to sort, furniture re-arrange, donate and declutter.