You need to stop comparing yourself to others. As you say, many of them are older and therefore are likely to have a much smaller mortgage or none at all. They might have received an inheritence. It is also nice that you acknowledge that you are reasonably comfortable.
One problem is that the standard of living that we have become accustomed to see as 'normal, not particularly extravagent' is only really affordable to a relatively small percentage of people.
If you think about things like:
Buying regular coffees/teas
Buying lunch at work
Eating out
Ordering take aways
Sending kids to ballet/drama/dance class
Shopping in the more expensive high street clothes shops
Getting nails/eyebrows/waxing done
Holidays
Replacing cars
Decorating and home improvements
People on average incomes, with average bills, can't really afford to do all those things regularly, even though things like having a coffee out seems so 'normal' and not particularly expensive, so we feel hard done by if we can't afford to have or do what 'everyone else' has or does.
Those that seem to do all those things all the time will either be:
Small percentage of very rich people (there are probably are disproportionate amount of these on Mumsnet).
People who are getting into debt and will run out of money sooner or later.
People who are subsidised by others, such as young people who live at home and pay little or nothing to their parents and spend all their wages on luxuries - again they won't be able to do this for ever.
However, some people work very hard at budgeting and getting deals, so seem to get a lot for their money - they may eat out a lot, but only on 2 for 1 deals.
You also don't always notice what people don't buy. Some people may spend very little on food, or cars.
I would try to ignore how others 'always seem to have more than you', because, while some do, its not always what it seems, they are not always 'happier' than you, and in most cases, it won't last for ever.
For example, I know someone who lives in a massive, beautiful house, they have two nice cars and always seem to have the finer things in life.
However, I also know that they are massively in debt with an interest only mortgage on a house that they 'cannot' sell, cause they need far too much money for it to pay off the massive mortgage.
They are both very close to retirement age and the husband has no pension, so will probably have to work past retirement age. The wife will get a reasonable pension, but it is tiny compared to their huge debts, but is in poor health so might not be able to work much longer (is currently on long term sick leave and may have to be medically retired). So while on the surface, they appear to be 'rich', I wouldn't swap places with them.