Why do I feel fat and depressed after going to the beach with my fit friends?

I'll start by saying that it's perfectly normal to feel like that. We all feel like that actually but as we get older we learn to deal with it differently.

I don't look as good as an 18 year old. Youth has the advantage when it comes to beach wear. Young people just overall seem to suit bikinis and swimwear better than late 30 somethings.

However. I'm old enough to not give a shit. I don't actually care what people think. I no longer compare my looks to those of other people as much. So what if they look good and I look frumpy?

I realise that this mindset comes from experience but that is the problem you're dealing with. You're equating fun and happiness with how you look. Try to enjoy the experience rather than the way you think other people experience you.

How should one live happily in solitude?

Generally speaking, It is not an easy task; but there are people who manage to. The purpose of Creation is our and God happiness; but long run happiness is not an easy task mainly for people elder than 50 years old about. To know more about happiness see Abstract 4 of Life Philosophy by Joao Ferreira.

Why are Spain and Italy said to be 'poor' countries?

Well there are differents reasons. I mean I'm not going to describe in detail facts we all know about Italy and Spain such as the fact Madrid, Barcelona, Valence, Milan, Rome, Turin, Genoa or Florence are much richer than many cities

How much does showering increase indoor humidity in homes?

Cooking, cleaning, and bathing contribute only a negligible amount of water vapor to a dwelling. Rarely much more than a pound/person/day. OTOH, foundation moisture can easily contribute 20+ pounds per day. And central humidifiers run off the chart.