Not all breakups are created equal.
Subsiding feelings on both sides is usually the "easiest" scenario. When you find out that you're only still together because you've gotten so used to it. This might still bum you out and it takes time to get used to being alone again, but it probably won't break you down.
Then there are the nasty ones: unreturned feelings on one side, certain "incidents" (cheating) and the IMHO most vicious scenario - strong feelings but external factors, often more or less mundane (like great distances), that ruin the relationship.
One should be cautious when hooking up with other girls straight after a breakup. When there are still strong feelings involved one might hurt other people unconsciously as intentions are not always in plain sight on both ends.