I'm trying to make mozzarella at home. I'm following [this][1] recipe (without citric acid), but I also tried [this][2] one (with citric acid), without any result. I think my italian is sufficient for understanding the above recipes. I'll summarize the video recipe for you:

 - I took 2l milk, (exact type below) and heated to 40°C
 - Add 1tsp natural yoghurt for each liter of milk (I doubled this)
 - Add the rennet dissolved in cold water (exact type below)
 - wait 1h, cut up the curd (so far seems to be ok)
 - wait 20min, till the curd and the whey separates (bit suspicious: the curd does not sink)
 - break the curd into small ("piccoli-piccoli") pieces, and let them separate for 4 hours (very suspicious: does not sink, floats on top of the whey)
 - remove whey, put the curd into a sieve, and let it drip-dry for 18 hours (suspicious: when it seems dry, I put it onto a plate for the remaining time c.a. 4h?, but the plate fills up with whey)
 - now I should be able to form the cheese in warm/hot water, but I do not get to this point, because the curd/cheese falls apart, it stays in little crumbs, like cottage cheese

What am I doing wrong? The materials I use:

 - [milk][3]: 3.5% fat, pasteurized (not UHT or ESL) and homogenized
 - [rennet][4]: c.a. 0.5ml for the 2l milk

I'm planning to try to get natural milk, only cooled, no pasteurization and homogenization, but before I do that, I'd like to know, what went wrong.

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Note: I've looked around here before placing the question, I found these:

 - [this][5] does not apply, because the milk I'm using is not UHT pasteurized 
 - the other questions are mainly about microwave recipes, which I try to avoid


  [1]: https://www.youtube.com/watch?v=0He5Ir7j3DU
  [2]: https://www.ilgiornaledelcibo.it/mozzarella-fatta-in-casa-ricetta/amp/
  [3]: https://www.billa.at/produkte/schaerdinger-vollmilch-laenger-frisch-35/00-425514
  [4]: https://www.amazon.de/Labextrakt-Premium-K%C3%A4selab-fl%C3%BCssig-30ml/dp/B00QSA1UAA/ref=sr_1_1?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=2UKKX7KEZK6IN&keywords=labextrakt&qid=1568481866&s=gateway&sprefix=labextra%2Caps%2C296&sr=8-1
  [5]: https://cooking.stackexchange.com/questions/70748/failing-to-make-mozzarella