gardening

All my plants die after they start growing and I don't know why. I've tried controlling every factor that I can although without a thermometer, higrometer, pH measuring etc. I even have a shitty microscope that I try to analyse the sick parts, but I can't find any reliable resources on how to actually interpret what I'm seeing. I want to know how to use this kind of data so that I can raise my plants right.

Where can I learn about this? I mean diagnosing problems, monitoring variables, finding solutions to each situation etc. google obviously sucks and gives nothing of substance

I will say that I recently got a new substrate, maybe the old one was the problem. But then there's my mother-in-law, who raises beautiful lavenders and all that using the exact same soil I'm getting shitty results with. I'm literally not doing anything different to her, so maybe it's the water? I really don't know.

Edit: in fact, the lush lavender 🪻 she is currently flexing is a piece of the one my partner bought. Same plant, same soil.

Edit 2: also, the roots always look alright when I dig their cadavers to analyze. No parasites, insects, obvious fungi etc in any part of any plant so far.