How you think really does affect how you feel.
This is why cognitive behaviour therapy works.
Depressed and anxious poeple have negative thouhgt patterns which make them feel miserable(depressed) and scared and worried about all the bad things that are probably going to happen(anxious, often hugely so.)
These thoughts are atomatic and have become a 'habit' and automatic pettern of thinking. This is why poeple believe you are either just a pesimist or an optimist.
In fact you can change your patterns of thinking and get out of a negative cycle that you beieve is 'just you', and poeple who do, report feeling better, happier and less anxious.
Take you example of miscarriage:
Two peopel find they are preganant. The optinist is happy and excited, aware of potential risks, but chooses not to dwell on them.
The pessimist, refuses to let herself be happy and excited and focuses on the possible negative outcomes incrasing anxiety and low mood, feelings of doom and hopelesness.
Opytimist and pessimist both have iscarriage.
Both are devasted as both really wanted thier babies. Both feel huge sense of loss.
Optmist howver begins to try and console herself with thoughts of trying again, liklihood of future success, stories of successful pregnancy and babies following miscarriage, which helps to lift her mood and lessen her anxiety about the future.
The pessimist, views miscarrage as proof that tings go wrong for her, and therefore things will probably go wrong again, so the future looks blaek, thereby creating feelings of despair and anxiety about her situation and future.
I'm noy suggetsing this is you Gordon, but just trying to illustart how pesimistic thinking realy does impact on your happiness and anxiety.
I really cannot stress this enough, I see it all the tme in my work.