How large is the space inside your shower, or is it a bathtub shower?
Also, how high is the barrier you will need to step over to access the shower?
I think these specifications are important to recommend the best safety devices. There is the possibility of bringing your walker into the shower with you, as I did, just in case I felt weak, although this is messy as it leaks water when you get out, necessitating absorbent towels on the floor, so I don't recommend this if you're alone it will be too much to wipe up yourself. I had the space to do so, but no shower grab bar aside from the shower door handle which I didn't trust because it obviously swings.
If it's a bathtub shower they do have medical adjustable bathtub safety rails you attach to the bathtub for safe and easy access in and out, but you will need the security of a wall grab bar once inside the tub. You can also purchase a slip resistant (stackable) shower step stool for easier access with higher barriers.
A word of caution, please do not trust adhesive grab bars, but only the type secured into the shower wall.