The thing is, you start to define yourself by nothing other than what you are to other people: mother, wife, milk jug. It begins with just realizing that you are constantly covered in spilled breast milk, none of your pretty things fit (between childbirth and breastfeeding), and your biggest thrill is when you manage to get out and take a walk with the baby. It builds up as you realize you no longer have a more personal hobby than snapping pics of your precocious 5 month old. Suddenly when someone asks you about yourself you can only describe yourself in relation to others. You've become that home made baby food you've been making lately: a mish mash of things that are no longer you, making you bland and somewhat un-appealing to anything other than an infant.
Now I'm not saying that your relationship to others shouldn't define a part of you; only that it shouldn't define you in whole. When you're a stay at home mom, and especially if, like me, you suck at it, it's all too easy to give up your own needs in favor of, well, in favor of anything else that comes up. You can easily begin to feel a guilt that you don't bring home the bacon, or at least a compulsion to do everything else, since someone else does that. We suffer from the delusion that if we don't do it it's unfair for anyone else to do it. It becomes simple to decide you can't afford to do something because that money isn't in your paycheck.
Before the good stay at home moms are up in arms, I am not saying that any of those things are true; only that it is easy to feel that way. And I do, far too often. I know how silly it is to think that Coffeeguy is keeping score, or that the mom police will revoke my license. But there it is. And worse, the very worst, is that I wonder if I asked him if even he could define me in terms that belong to myself, that define only me. I can't bear to ask, so I guess I'll never know.