My baby girl (1year 9 months) has severe nappy rash. what can i do? ive tried bepanthem, e45, fissan paste. they generally helps a bit, then returns

Some basics: These rashes reflect the moist environment & the breakdown of the skin barrier by that moisture with irritation and invasion by poop germs/yeast. All the products you mention have some benefit by providing a skin barrier to those conditions. More frequent changes, water rinses and blowing the skin dry (hair dryer-no heat setting) helps. Any red bumps are likely yeast & require an anti-fungal.