Can't join most servers

For some reason, for a few months now, about 3/4ths of the servers I attempt to join return a “server is not responding” error. It happens on all gamemodes, and even if I attempt to connect through the console. I’ve tried deleting clientregistry.blob, but that only works once, and then when I disconnect from the server, I have to restart the process all over. Does anyone have any idea what to do?

First thing that comes to mind is anything that would have a firewall or act like a firewall.

I already have Garry’s Mod set as an exception in Windows Firewall, unfortunately.

Perhaps do you have a router that is blocking the port?