Don't sleep with someone else just yet
So, you've decided to cut her loose, and you're not going to wait until after her birthday/Bastille Day/winter. Kudos, sir; you've done the right thing. However, it is worth noting that you're not completely unmoored, and you are not yet entitled to guilt-free sex with other people.
Sure, give yourself a little space in the days leading up to dropping the B-bomb (like the L-bomb, only with sad tears, not happy tears), but don't take respite in another girl's vagina just yet. That's the quickest way to guarantee an emotionally sloppy breakup. And although it does clearly say "I'm no longer interested in our relationship," there are better ways to communicate this, ways that make you look less like an asshole, and more like the gentleman you are.