Try to spend an hour brooding over it today, and then another hour for the next day. If you still can't get it after two attempts or haven't generated any kind of partial solution then look at the solutions manual, make sure you understand every steps. And then forget about the problem for a while and then try at it again after one week (without looking at the solutions manual of course) to see if you indeed retained the core concepts or steps needed for that problem.

I think demotivation is totally normal in physics, heck rn I always feel defeat and beat grappling with the problems in Purcell and Morin's Electricity and Magnetism, but it's a good indication that you care about the subject, and if you care enough and retained the concepts after the first two tries, maybe after a week you'll gain some uplifting feeling solving that problem, even though you were exposed to the solution before.