TL;DR title.

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I believe there are two different types of psychological reactions when a woman leaves you.

Reaction type one. You miss her. She added to your life and now you need to provide it to yourself, if you want it. You give it some thought, you mourn your loss, you analyse your loss and then it goes away. A few days maximum. Then boom, it is solved. Panta rhei, que sera sera, whatever, man.

Reaction type two. The pain is way more intense, the oneitis is strong, you do a lot of crazy things, you try to take her back, you ask at Ask{M,T}RP how to get "that one back", you get your ass kicked in the comments, you get called names, you get banned, you delete your posts, you crawl to her on your knees to see if she will take you again (she won't, never ever), you cry, you think about killing yourself, all this crazy stuff.

Reaction type one is caused by healthy bonding mechanisms. You somehow managed to create a bond with her, she left, it hurts, it will stop. This is a proof that you're a healthy male. Only guys acting in a passive-egressive ways will try to act "macho" in such situations, at least in front of the mirror they will. Feeling loss when you had some loss is normal, at least it should be.

Reaction type two is caused by finding yourself a mother replacement and actually never cutting the umbilical cord. It hurts, because you're growing up. You rather think about committing suicide, because life without mother seems too scary. All those crazy behaviours, all this thinking comes from a very early stage of life, a stage that you haven't actually lived through, somehow you managed to reach 20-something, 30-something, 40-something age without actually separating yourself from your mother. How old are you to think you won't be able to survive without mommy? 2 years, 3 years? Earlier maybe?

The pain and the fear are outstanding. Ex-marines may laugh now because why not, but anyone who has been through it will understand. The pain and fear in that phase come from very early stages of development and controlling them in the beginning can be hard.

Summary

Most of the hurt and pain of MRP newbies, myself included, comes from type II reaction. I think it can be also called "growing up". A support from an established group of males who can relate is crucial in this phase.