Not knowing the numerous walkthroughs you executed, there are a few things you can try (which you may have done already.)
The easiest is to remove all the Preferred Networks from System Preferences > Network > Wifi > Advanced button. Select each network and click the minus symbol. Then turn off wifi and restart it. Lastly, re-join the network you are having problems with.
You didn't mention whether you restarted the wireless router. This can sometimes help.
Lastly, are there any other wireless devices that you use (iPhone, other people's laptops, etc) which have the same problem?
Please let us know what troubleshooting steps you've taken already.
Wifi problems can be maddening. You're not alone.