Probably a lot of guys came to this site after being dumped by a girl, and I'm sure ALL of them naturally toyed with the idea of trying to get her back. Here's my thoughts:
A lot of people believe that once a girl's interest goes past a critical point, you're out forever and she will never become attracted to you again. This is not the truth in reality.
If you've been with a girl for over 6 months, she has a certain amount of investment in you. She desperately WANTS to remain attracted to you, and it DOES pain her to end things.
At the same time, she expects you to behave like a typical AFC and beg her not to leave, promise you'll do anything, whine to your friends, and ultimately grow anger and bitter towards her. This, in her mind, is confirmation that she did the right thing.
If you keep your cool, accept the situation, and DON'T CONTACT HER, she will, 9 times out of 10, call you in 2-3 weeks to "see how you're doing." This is because she HASN'T received the confirmation as I described above. If you still remain cool and congruent, you can rest assured that her interest in you will rise up again--EVEN if she "branch swung" to another dude!
It helps, of course, to begin dating other girls, but even if you aren't, this lack of interest in getting back together on your part will crush her ego. She will likely now actually begin to try to "win" you back by asking to "catch up," or bumping into you "accidentally," etc.
At this point, many guys are EXTREMELY tempted to give in and take her back, believing that she's seen the error in her ways. Or, they might believe that they can get "revenge" on her by using her as a FB and then dropping her when they find a "better" girl.
BOTH OF THESE THINGS ARE BAD DECISIONS!!
You can try to fool yourself into believing you can have no strings attached sex with this girl, but you can't. You will start feeling attached, and give her the benefit of the doubt when she tells you WHAT YOU WANT TO HEAR so that you'll forgive her.
"I was just confused"
"I've missed you so much"
"I realize that I belong with you"
...are all popular lines. Unfortunately, they don't mean anything when said by a girl that desires the validation of knowing that she can get you to take her back.
If you DO take her back, she won't necessarily do it all again (although the odds are pretty good that she will), but she will have the knowledge that she could. She will uphold the fact that you took her back as a great quality of forgiveness that you possess, but in reality she's just satisfied with the knowledge that you'll always be a "safety net" to fall back on.
Furthermore, the dynamic of the relationship will change now that the cat is out of the bag, so to speak. Initially, you can expect her to do things consistent with a VERY high interest level.
"She came back to me and now she's into me MORE than ever before!" are the gloats you hear from many guys. This is only a temporary spike in interest and doesn't really reflect how much (or little) respect she has for you.
The worst part, though, is the subtle feeling of resentment that you are bound to feel, should you decide to take her back. You may NEVER be able to fully trust her again, and even crave revenge. This is not a good way to go through life.
The reason we advocate spinning plates after a breakup instead of trying to get the ex back is not because the latter is difficult to do. It's rather simple if you are capable of not wearing your emotions on your sleeve. However, it's healthier in the long run to start fresh, with a new girl, with no negative emotions involved.
Would you truly feel comfortable getting married and having children with a girl that broke your heart? You need to answer that honestly. I personally believe that we are ALL entitled to a "perfect relationship" with no breakups or cheating.
I have the same feelings about cheating being a dealbreaker in marriage, UNLESS children are involved (When children are involved, you need to think of how such an event will impact their lives at such a crucial stage in their development). Marriage counseling and attempts to "get past" infidelity are notoriously ineffective. It's simply easier and healthier to start fresh.
Keep in mind that NONE of this is meant to suggest that women are not to be trusted, or have poor intentions, or anything of that ilk. Most of the time, breakups are actually OUR fault. It's up to us to keep their respect, and if we fail to uphold our end of the bargain, they often have no choice but to leave.