This is easier to understand if you break it into phrases.
Don't let
what you can't do
stop you from doing
what you can do.
It means: don't let frustration, fear or disappointment turn you off. Use your sharp eye to find what opportunities are still available, even if they aren't the one's you were originally aiming for.
Many successful people started with one goal, were blocked by one thing or another, and shifted their goal to something that turned out even better.
Yes, it is your life and if you don't like what they are doing CHANGE IT (but if you want to stay with them then say it politely. Of course you can, but that does not mean he will stop doing it. Depending on what 'it' is then you not liking him doing it may be none of your business.
Keep Doug it
It means that the love their child; however the child may be doing things that they do not like and wish they would stop doing.
stop doing it
you cant do anything men, admit it your in love with her
It depends on if you let them win or not. Sometime they do because no is there to stop them from doing so. So if you wanna cheat (dont) but if you do, do it secretly. Which i mean dont tell everybody you know you did it.
stop killing them and dont destroy there habitat
It basically means stop doing whatever it is you are doing, but can also mean to chill as in hangout.
Superbad-is-a-fictional-comedy-dont-take-it-seriouslyism
You cant that is basicly your computer working most new computers dont humm but no you cant
Dont you mean 'how'?
In the UK we would use the phrase 'Knock it off' to mean stop teasing/irritating me or stop what you doing.