Some days I walk into the gym with stuff on my mind and frustrated with things I can't resolve. Without fail, getting in the gym, moving, and the feeling that you've achieved something in the gym helps to get rid of that negative energy. I tend to look preoccupied when training even when I'd do solo sessions at CrossFit gyms. I do a lot of thinking while at the gym and training on my own.
There may be stuff still to resolve but you've had some restbite, released some positive hormones, and will possibly come out with a new outlook on a problem.
Things that made me happy today..was squatting with a little more weight today with no pain in my lumbar/pelvic area..WIN! Hitting the biggest weight I've ever done on hamstring curls (hamstrings have got a lot stronger while rehabbing). My back also coped with my GHD hip extension sets going up to 14 reps...WIN. Then in the video played around with 3 x 12 GHD (Glute Ham Developer) partial range sit ups...WIN.
So when the frustration builds...we have to find an outlet for that and put the energy to good use. I forget sometimes...sure we all do..but the remedy is often staring us in the face!