I am of the opinion flatwork is meant to be boring, that's why there is no "competitive flatwork". I guess dressage is similar, but to me there is a difference between flatwork and dressage, but I don't like either. But I am firmly of the opinion that its neccasary for control and to be competitive.
I'll share how I approach flatwork. I don't do it for over an hour, horses do not concentrate for long periods of time and few horses are going to concentrate for an hour of flatwork. Decide what you want to do. I don't think you need much longer than 30 minutes of actual work. I would approach it as - warm up time - 20 - 30 minutes of work time then cool down time. Look at what you want to work on for that session. Decide where you want start. Is your flatwork for teaching the horse, or are you just doing it to maintain his current training?
If you want to do circles then decide to start at trot 20m circles, and then work towards acheiving whatever you want. Don't expect too much. I've had those sessions where I wanted to perfect canter transitions or something, and then gone way over the time I would normally ride obsessing about it, and my horse loses attention, gets tired, and I get frustrated. Know what you want and make sure its realistic. If you start out and discover its not realistic, then change your goal mid way through but I have a firmly set goal so when you have finished your flatwork session you can walk away with an achievement, and it suddenly won't feel as bad. Compete against yourself.
Do something a bit different, who says you can't practice turning around cones or something. Set out poles to practice transitions in between. Do a whole session bareback and practice your rising trot. Do circles and transitions without any rein contact. Teach you and your horse something that you don't know.
Don't over do flatwork. Work out an equation for yourself. Maybe 4 hours of flatwork equals one hour of jumping and one hour of trail riding. Or something like that. Keep to your plan.