Are the following sentences natural to a native ear?
1. I thought I would give you some time and you guys would be mature enough to work this out on your own.
2. I stretched myself before the game.
3. That account has a thousand dollars extra.
I will only add that the construction -- verb + myself -- is used with much less frequency in English than in other languages. The main reason is that in English 'myself' is used to emphasise achievement of something without help.
"I baked the cake myself!" (I did it without help -- not I baked the cake to myself)
"Susie learned how to tie her shoes herself." (She learned alone, without help).
There are some exceptions when 'myself' indicates that the subject and the object are the same, but they are limited to a few situations. These include:
"I thought to myself, 'how can that be right?'"
"She's talking to herself. Are you sure she's OK?"
"A key lesson in being loved is learning to love onesself."
When I moved to Canada, my relatives had a good laugh over me saying "I lost myself" instead of "I got lost".