"A schedule defends from chaos and whim."
I have had a great deal of success over the years as the result of scheduling my practice sessions.
To be clear, I don't mean how long they are to be. I find that when my primary goal is to practice for one hour instead of improving specific musical passages, I get bored easily.
By scheduling a practice I mean two things: when the session will start and what will be accomplished.
I encourage all of my students to never end a practice session without going on the record with yourself about exactly when your next practice session will begin.
You can always reschedule. But it is a lot harder to skip a practice session that I've already scheduled (even with just myself!) than one I haven't.
And never begin a practice session without having a very clear idea of what you are planning to accomplish. (Hint: "Make Mozart's Third Horn Concerto better" is not a clear idea!)
I will choose four measures in this piece, three pages in this book, and a page of this solo and will put them in a stack on my music stand. I then don't get up until those goals are accomplished.
If you've never scheduled your practice sessions and planned specifically what you are trying to achieve in each one then I challenge you to try it.
You will be impressed by the results in a short amount of time.