Arthur, first welcome to the site, second congratulations on the 300 game and 782 set.
I have a question for you, How much of the bowling bad was you being down on yourself instead of realizing you were bowling just as well as you did in the 782 set except instead of strikes you got 9 counts, you were still doing the same things, and how much was it bad luck or you getting robbed?
The question you have to ask yourself is the next time you aren't getting the results you wanted, try to figure out why? Instead of thinking of how you got robbed or are just unlucky, when in fact you are probably only a couple of moves away from good scores, try not getting upset and frustrated and figure out what the ball is doing and how it is hitting the pins.
It is best not to think about your 300 the next time you bowl, just try to remember how the ball felt coming off your hand and try to repeat that good feeling you had. Visualize the ball striking the same way it did the last time and it will happen for you again, you did it once, you can do it again. If you miss, you are only human, just make the spare and get back to business.
I am just trying to point out how much a bowler's attitude has to do with scores. I wish you luck, if you need help figuring out what adjustments to make, let us know what line you were playing and what spares you were leaving and what ball you were using.