with the occasional steel wool
Don't do that! It's likely what's killing your tips. Anything that might scratch the surface should not be used. That includes steel wool, sandpaper, files etc.
Is there any way to save this tip?
Looks like you've killed it with steel wool. Get a new one.
You can use a damp sponge or a brass sponge to remove solder from the tip during soldering. To remove oxidation, it is usually sufficient to bathe the tip in (rosin core) solder.
For the occasional deep cleaning of a badly oxidized tip (or in case you've made a mess by getting melted plastic on the tip etc), you could use something called tip activator. You just dip the heated tip in it, works nicely.
As mentioned in another answer, you can increase the the tip life by covering the entire tip in excessive amounts of solder just before turning the iron off. This prevents oxidation.
I set the temperature to around 300 C
Not a problem. 350° is the recommended professional setting. Depending on how bad your iron is to transfer the set heat to the actual tip, you could turn it up to hotter temperatures still. I have to do this on bad Hakko irons for example. Whereas a premium iron transfers heat faster and better to the tip and you actually get the temperature you have set.
Since lots of answers here seem confused:
Do not mistake the solder melting point temperature (~180°C leaded, ~220°C RoHS) for the temperature of the solder iron. You aren't supposed to melt the solder with the tip. You are supposed to melt the solder with the heated metal surfaces where the joint should be formed. Therefore the tip needs to be significantly hotter than the solder melting point temperature or it would take forever for the surfaces to reach that melting point.
