How do you make yourself stay in mental health and physical health before other needs?

You need to allow time for fun for doing things that are fun with your family and with your friends. This is more important than material things. You have to know that life is not all obligations and work but there are enjoyable things to look forward to even a hobby can be helpful.

actually "as we grow older and older" we are less likely to sweat the small stuff or have to worry about dealing with children since they are grown. my family and my husband's family have never much bothered with each other and that is fairly common

Answering the first question:
- physical health is obviously important. We feel the results of self-care in our body and quickly see it in the mirror.
- mental health is probably maintained through spirituality, Faith and connection to God. I may be wrong on that one, so don't take my advice 

Dealing with stress is a very individual thing.  Different things trigger stress in different people.  For a start you need to list all the excellent things you have in your life and prioritize them.  List all the things that stress and upset you and figure out ways to to cope.  Check out Mindful Thinking, Meditation and breathing exercises.
And if all fails, seek mental health counseling so you can be helped in achieving a less stressfull state.

Alcohol in moderation to dull the mind a bit. Pipe tobacco to slow down and relax. Finally, fill any large blocks of idle time with something interesting.