Oh dear. This is a very classic example of a thread that would have gone another way altogether had the OP been a woman.
I do think that men sometimes don't do themselves any favours in the way that they express their feelings and describe the issues on MN - this so clearly is written by a man, the language is different. Men often write/speak in a way that makes them sound more offhand but it doesn't mean they are not feeling just as hurt inside as any woman who might write about the same problems. Some of the responses on here are very unfair and undeserved and the same things would not have been said to a woman.
TheOtherSide I really think it's crunch time. Whether it's because of her depression or whether she just doesn't love you any more, you need to bring this to a head now and force a conversation about it. Find out what the problem is and whether she is even remotely interested in fixing it, or whether she is actually just hoping that if she keeps this up you will admit defeat and go away quietly.
Things have clearly gone too far to keep trying to 'be normal' You've tried that - it doesn't work, for whatever reason she is really struggling with facing up to what's going on in the relationship and she's burying it. Maybe she's frightened that if she lets it all out things will never be the same again and she just wants to keep the family unit together because she's scared of the alternative. That can work to a point, but only to a point. Eventually, if one person in the marriage is really unhappy it will be like a pressure cooker that will blow.
Stop trying to coax her into normal sociable conversation and then get frustrated and angry when she rejects you. It's understandable that you feel immensely frustrated and want to vent about it, but she obviously can't deal with the confrontation and just wants to close it right down, which is why she walks away.
There is only so long you can go round in that loop. It's time to ask her straight what she wants. Tell her calmly, gently but firmly that you are both GOING to sit down and talk like adults about about the state of your marriage and its future.
Pick a time when the children are in bed or at granny's, don't blind side her at a totally inappropriate time when she's busdy or distracted, or on the the back of another row or rejection, or when emotions are running high, that will be a disaster.
Ask her straight what it is that she wants. Ask her if she feels insufficiently supported with the children. Ask her if she can articulate what it is that has changed for her and if she even knows why she is unhappy. Ask her straight if she has simply stopped loving you and wants out, but can't being herself to say it. Ask her about her depression and whether she feels overwhelmed and unable to respond to you in the way she would like, but still loves you. Tell her you just can't carry on in this weird state of denial, in limbo, being unhappy and confused and not knowing where you stand.
It's tricky because if she knows you wont leave unless she forces you to then the ball is still in her court really, and she can carry on with her state of avoidance and denial for as long as she likes and you'll have to lump it. Maybe she's secretly hoping you'll leave if she keeps this up for long enough, so she doesn't have to face up to being the one to say it's over.
But hopefully the 'make or break' conversation will galvanise her into action or an honest response of some sort - even if the results are not what you would hope for, or you have to sit there and hear a long list of criticisms about you and why it's all your fault. At least she'll be talking and you'll know here you stand. And you can move forward from there, either alone or together.