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Hi Adam, as a Mother and a Wife of 18 years it could be any number of stresses ranging from the safety, welfare, health and happiness for both of your Children to being rundown emotionally and physically, particularly if your Wife is now in her late thirties or forties; following that is Menopause, lol. Its a known fact that a Wife and Mother take on all of the stresses of their partner, the stresses of their Children and lastly if there is any time or space remaining, some of their own individual stresses may be looked at or acknowledged; rarely addressed.
I would be ensuring your Wife is taking all of her vitamins, iron, minerals and supplements, de-stressing by taking walks or attending the Gym, swimming or whatever exercise leisure time activity your Wife prefers. Next, I would be ensuring that you all dine out at least once a week to ensure that your Wife has a night off regularly (you may share the cooking already or do bbq’s, either way it is getting out of the home together for some relaxation out of the home)!
I would be sitting down when you are both alone and before she starts on any whingeing, anger or argument; ask her genuinely why she is not happy and meet her half way if it relates to household things. If it does not relate to household things or yourself personally as a Husband; it is all within herself and she needs to address the areas of anger/resentment/insecurity/ whatever it is, that on many occasions stems from our childhood issues.
She may wish to take up some art or creative classes such as craft, knitting, crotcheting, music, dancing, learning an instrument, take up writing, anything which is relaxing and therapeutic and initially gets her out of the home.
Taking time out to read is a wonderful aid along with writing.
Love and best wishes to you, your Wife and Family