#1
There's a cavity forming on one of my frets on the B string, and it catches B, G and E as well, is there anything i can put it in to fill it or do I need to get a professional to put a new fret on it for me?
#2
No filler available. Cavity sounds severe. Sounds like at the least a fret dressing is needed, or a new one.
#3
What guitar is this?

Fretwear is a normal occurence, and it usually happens in places where you like to play the most. If it starts becoming a problem your guitar will need a fret leveling + crowning. It can be very pricy, but there is no other way to fix it. Re-fretting is even more expensive. If it's a bolt-on, you can get a new neck.

American Stratocaster + Blues Junior

#4
It's my S, it's like someone got a knife in just put a slit on one of the frets. I guess I'll just keep an eye out for a new neck on ebay, thanks dudes.