What can I use in my shower to prevent falling?

Get a shower stool or chair. Inexpensive and sold at big box stores and on line. I have both, needed them after I had surgery for the same reason, did not want to fall.

I installed grab bars. They now have nice looking ones that don't look like hospital equipment. I will never be without one in the shower/tub plus one to grab when I am climbing in and out. You can also throw a small hand towel on the floor of the shower to stand on.

Buy plumbers Goop and put in straight lines on the floor.

Until you can get grab bars installed properly, you might want to get a pair of these water shoes to wear in the shower.

They are called a variety of different names depending on where in the world you live- aqua socks, ocean shoes, water aerobic shoes, etc. but are all similar design- neoprene or mesh body, and rubber soles for grip, no slip. They are commonly recommended by physiotherapists for people participating in water exercise classes to increase their balance and mobility.

It's totally your water softener!!!

I've had water softeners installed in homes with textured tubs and showers, with no noticeable difference in slipperiness.

There is no industry standard of consistency as to what "textured" really means. Thickness and finish can vary greatly from brand to brand and it can disintegrate over time.

Perhaps you have soap residue that does not completely rinse out. Try wiping it with vinegar after you shower to cut the soap residue or greasiness left over. You could try using marine silicone to keep a mat down on the floor. It is made to be used in water, that is what keeps the aquarium sides together.. If you do try it, make sure the floor is clean and dry and follow the directions for cure time so that it will stick. It stays pliable and should be able to easily take it off with a plastic scraper.

I use a stool in my shower to keep from slipping & falling. I had a suction type grab handle that I had to use once to keep from falling and the suction came loose, so it and I fell out of the shower. If you decide to have a grabber or two, please have them permanently installed.

Can you etch it/scratch it with a sharp knife or nail?

Stephanie has a very good point to not use the suction style grab bars. These are a cheap unsafe false alternative to the real sturdy correctly mounted grab bars.

Previous owner of my home installed a handicap bar in the shower. Now 20 years later, I appreciate it. You'll need a professional to install if it's a ceramic shower, or a drill with a bit that will go through the ceramic or stone. Be sure to caulk around screws to avoid water getting behind device. Got a built in soap dish? I've grabbed that in a pinch in my mother's house.