I'm trying to make mozzarella at home. I'm following this recipe (without citric acid), but I also tried this 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: 3.5% fat, pasteurized (not UHT or ESL) and homogenized
• rennet: 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.

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

• this 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