Use some tape to get straight clean lines. Keep it tighter to joint. The thin extended edges will not hold up to normal use a cleaning. The tape should be removed before caulk sets, but wipe it with finger dipped in mineral spirits. They have some tools for getting nice finishes that work well.

Oh, you need to watch this video. He points out all the reasons your caulk job failed. I used his method, which relies on a caulking tool, and my tub looks new 3 years later.

Basically: Tape pulls up the edges Using your finger produces these this layers that peel up. Misting soapy water lets the water run behind the caulk and it doesn’t stick.

https://youtu.be/_DI4hfHM_Hg

Here’s my one additional truck to keep it from being messy: cut a bunch of paper towels into 4” squares. Put them by your knee as you work. Also have an open-top trash can with a liner. When you wipe a section of the caulk with the tool, you’ll need to clean the tool before swiping the next section. Use one corner of a square; then the next, etc. this makes it easy to tell which you’ve used, and you can drop it in the trash can when you’ve hit all areas. (Using a wet cloth was so messy because I couldn’t tell where I’d wiped already.

Also—he suggests a caulking tool that has lots of nifty features. You can get less expensive tools without them for about $8 or $9

What my tub looks like: https://i.imgur.com/pWA4roC.jpeg

You want to use silicone caulk.

Three things: First, the surface needs to be completely clean and dry. Second, it needs to be the right caulk- for shower and tub, 100% silicone. Third, lay the bead correctly- it looks like the previous bead was too wide and thin. There are some tools available to help, like this

Your caulk was likely bad. Could have been an old tube, a tube that had a leak so moisture got in, a tube that was poorly formulated, or the temperature/humidity wasn't right when you applied it.

The tub probably has a residue on it that caused the caulk to not stick well. Or, there was still existing silicon caulk on the surface and new silicone won't adhere to it.

You'll need to remove the silicone using silicone caulk remover than clean the surface really well with some rubbing alcohol before doing it again.

Most likely you didn't prep the surface we'll enough. You will need to scrape off as much as the old silicone as much as possible. New silicone will not stick to the old silicone well. You can use a utility knife or a razor blade. Acetone the area and then apply the new silicone. Don't use the shower for 24 hours.

Caulk doesn't flake.

You installed it in a sloppy fashion.

Fill your tub with water before you chaulk. If it was installed without proper support it could drop enlarging the gap. So every time you fill it stretches the chaulking causing it to breakdown much quicker.