How do I search my command-line history for commands I used before?

Press Ctrl+R and type ssh. Ctrl+R will start search from most recent command to old one (reverse-search). If you have more than one command which starts with ssh, Press Ctrl+Ragain and again until you find the match.

Once you've found the match you can press Enter to execute the command or left / right cursor to just select the text of the command.

There is no default reverse option for Ctrl+R to invert the direction of the search but here you will find some suggestions about it.