Well, please read the instruction on the box...
There are leave on serums
Others are rinse off
If it says rinse off pls do because you will be doing more damage by not following instructions
If its the Pantene 3 minute Miracle, you are supposed to rinse it off.
If you do not intend to rinse it off, then you only apply the serum to the ends of your hair, not through-out. Do not apply a lot, because serums/creams or whatever will load your hair down and probably by means of gravity, drag your hair off. (Just scaring you.)
And if you hair is totally fizzy with white hair. Consider using the Garnier range of hair colours, I find that the dye don't dry my hair, give its a nice dark colour and the conditioner works really well to smooth out the fizz and dryness.
After that, do consider apply olive oil to the ends of the hair, or at least the hair from the ears down and sleep on it. Maybe a towel on your pillow. And wash your pillow often.
You also should make sure your diet consist of plenty of olive oil or fish oil to keep your scalp and hair healthy.
Major sickness or diet will affect the thickness or breaking of your hair shaft.