How do I block myself from using Safari?

You cannot invoke parental controls on an admin account. What you should do is to create a new user account, then make that an admin account and make your present account a non-admin account (uncheck 'Allow user to administer this computer').

Now you can set up parental controls on the account (which obviously you would do from the new admin account); in the 'Web' tab you can cusomize access.

You should create a fairly complex password for the admin account, write it down, and put it in an envelope upstairs. Obviously since you are the administrator you can't completely prevent yourself from accessing these sites if you are determined to do so, but by making doing so entail a trip upstairs and a change of account you can at least discourage that casual 'just for a moment' temptation that leads to a long time-wasting session (we've all been there).

There are also third-party parental control programs, such as Integos, which are more configurable and could be set to block certain sites or even sites containing certain words, even when you are in an admin account (this may be possible on a timed basis but usually it's overall access which is timed) - but ultimately they still depend on a password. However since you could set it only to block these sites and do nothing else you could I suppose get someone else to set and keep the password, though this could also get inconvenient. (You could, again, set a long un-memorizable password, write it down, lock it in a cash-box in the loft, put the key in a place which isn't too easy to access... you get the idea: it would make it not worth the bother for casual access).

The problem of course is that all these methods are designed to prevent your kids from accessing the sites, not you.