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.