[With acute awareness of what mental health disorders are, and what they are not, I say sarcastically:]
Well, we ALL have 'mental health issues' when we're married to abusive husbands! Often times, they tell us this...and their behaviour makes us think we are going mad.
So do they: but we've defined that as "cognitive arseholeance" - a unique condition of abusive husband and their cognitive dissonance!
It's a little chicken and egg story if ever there was one! That's why divorce is good for us: Instant cure (once you get to the end).
Quite frankly there are bone fide mental health issues, e.g. depression, schizophrenia and some personality disorders. These are are well documented, and can only be diagnosed by a suitably qualified individual. Dr. Google is not suitably qualified. Neither is the layperson. For those with genuine mental health disorders, we should offer empathy and understanding; their condition will not have a definitive end date.
Ours do. Ours are a matter of not agreeing/or seeing it through their eyes. It is a matter of a difference of opinion or perspective. In other words: perfectly usual in divorce.
I frequently say my STBXH is mad. I am not suitably qualified to diagnose him, but I cannot fathom his way of thinking. But he is divorcing me, ergo I have little pleasant to say about him and his behaviour.
But that is my spousal opinion and nothing more.
So my advice to you is to get your divorce any which way you can - and that means accepting what he writes. Divorce is not about an individual's behaviour, it is not like what you see on the TV whereby your characters are on trial, it is more about making firm arrangements for childcare and for dividing up the assets.
In other words: that one person wishes to dissolve the marriage is accepted, however, the kids and the cash need to be taken care of. The children's needs are the most important consideration in divorce.
If he is suggesting that you have a mental health disorder in order to get custody of your children - then that is a whole different ball game. Assuming you are the primary carer, you will have to be clinically assessed (court ordered) and before that happens he will have to have very good reasons for making this allegation (well above his opinion), e.g. you have previously been sectioned, or have on-going social services involvement because of your mental health. Judges are very familiar with these kinds of allegations, whereby the children are used to punished/threaten the other parent. He should tread very carefully before trying this on. However, even if you do have/or have had a mental health issue (e.g. post natal depression), there is nothing to suggest that you will be deprived of your children.
And hell - if arguing with your children was that bad - none of us would ever be allowed to keep our kids. There is unacceptable treatment of children and there is parenting children that sometimes drive us crackers!
A good book to read is Family Law by Gordon and Slater, which explains the divorce process in layman's terms, and has several chapters on how children feature in divorce.
It is a very clinical: Divorce is the most unemotional of processes. Come here to vent (I do), but don't confuse feelings with the cold calcuated reality of divorce law. They are not connected.