Those wet solder sponges cause a thermal shock every time you wipe the tip. I recommend using brass windings instead. They also won’t completely wipe the solder off the tip either.

You always want a small layer of solder on your tip at all times to prevent it from oxidising. Oxidation means bad heat transfer causing soldering to be much more difficult. Also store it with a small blob of solder on it.

You should avoid scraping or sanding the oxidation of the tip. There is a small layer of silver that won’t oxidise so quickly but below that is normal iron. Not sure if that applied to cheaper Chinese irons but it’s better not to learn bad habits. Once you remove the silver layer you’ll get oxidation much more quickly and you’ll have to keep scraping until the tip is gone.

You can remove oxidation with the brass windings much more easily than with a sponge. When you apply solder with flux the rest of the oxidation should come off fairly quickly.

Buying a cheap model to practice is a great idea, when you are used to that you can look into a better iron. I’ve been very happy with the TS100 but the Pinecil is the newer model apparently.

I think it might be a controversial topic, but I’m fond of tip tinner - something like this:

https://www.amazon.com/dp/B00NS4J6BY

Sponge may be burning as well. Do you keep the sponge moist?

The trick is to keep soldering tip always wet from solder and only clean it just before you solder. Second, always when done soldering and you cool down you soldering iron, melt fresh solder on the soldering tip before it cools down.

Probably the equipment or maybe how you’re touching the part you want to heat.

Take a look at this video where Louis Rossmann explains which tips works and which ones don’t and shows why.