Nail varnish contains plastic (making it stretchy, flexible and shiny) so when it flakes off it adds to the microplastics problem in the oceans. Plus it requires a host of plastic to use - plastic bottles of remover solution, plastic lids on the nail varnish bottles... The varnish does come in glass bottles, but they are impossible to recycle as they are coated in residual nail varnish, so these bottles end up contaminating batches of recycling. Nail varnish releases unpleasant vapours (as does the remover) and prolonged use damages your nails leaving them yellow and brittle (which then makes you want to wear more nail varnish to hide these issues).
Nail varnish applied beautifully makes you feel great. It is very hard to wean yourself off using it regularly. Especially if overuse has left your nails weak and damaged.
If you do decide to give-up nail varnish for the sake of the environment, you'll need a nail polishing / buffing block (the one in the photo is from Nails Inc), such as this one from The Body Shop for £4. It will polish your nails to antural shine, especially if you rub a little coconut oil into them too.
Good luck with this step, it's a tough one!