Sorry about the disappointment. It seems especially terrible when our tomatoes don't survive as well as we expect. Can you obtain Bacillus thuringiensis, also called Bt, in Denmark? (It might also be called a "natural caterpillar killer.") If so, use it on a regular basis. It kills only the caterpillars less than half grown, but I suspect that's mostly what you have. The truly good news is that you can use Bt even on the day you harvest. Don't beat yourself up about this event. We were all beginners at one time. (Even me!) With time, you'll be helping other folks.

It looks like some sort of larva being parasitised by a tachinid fly. Here is a link that might be useful: Tachinid Fly

i second BT...Bacillus thuringiensis go to your garden center and get a bottle of "caterpillar killer." that's BT...it's a bacteria that acts on the digestive system of the caterpillars and kills them...but is totally safe to us. it's organic...and you have to spray every seven days or so...but (for me) it beats having to hand pick if there is a heavy infestation. although...if they are only eating leaves and they are just an annoyance more than anything...then i would just hand pick. admittedly...i hate touching squishy caterpillars...(heebie jeebies) ESPECIALLY horn worms!!!! they're SO huge...they seem almost...animal.

Fungus gnats look like this and the way they fly is somewhat similar to mosquitoes, although they'll be faster than mosquitoes in warm weather. https://www.maximumyield.com/fungus-gnats-the-cause-the-problem-and-the-answer/2/2087 If you look in your mix in your pots, you'll see their transparent maggots. https://static1.squarespace.com/static/57f32906f7e0abe5df5634f9/t/599a404ff7e0abf8e325a4bd/1503281241172/fungus+gnats+larvae?format=750w Winged ants, OTOH, look like ants with wings. However, after watching your video again, those really look like fungus gnats to me. Very lively, super happy fungus gnats, but definitely fungus gnats. You can deter fungus gnats in drier conditions by keeping the substrate as dry as possible and trying strategies like putting an inch of sand on top of your pots. But with an infestation that's already this bad and in this wet weather? Forget it. Get the mosquito dunks. They're non-toxic to wildlife and, just as BTk only affects caterpillars of moths and butterflies, BTi only affects the larvae of long flies like mosquitoes, blackflies, fungus gnats, etc.