Besides Making milk cheese, milk lotion and milk soap (techically, what you can do with cow's milk, you can do with human milk, I suppose), here are some easier way to make you feel less guilty over having to waste milk, and to zero waste.
Nourish the Plants
Add your breast milk to your collection of waste water for the plants.
Especially if you grow vegetables or fruit trees. You get to reap what you sow.
Feed it to your dogs and cats
And so I read somewhere that people do that. Well, if human can drink cow's milk, surely dogs can drink human's? But you probably not want to give them expired ones. Be humane or animale.
Carousell your Breast Milk
I. Am. Amazed.
Donate your Liquid Gold
Although there are no official human milk bank in Singapore, the global movement of Human Milk 4 Human Babies has its Singapore arm, called Human Milk 4 Human Babies - Singapore, where you can donate your spare milk to other mothers (or rather, their infants).
Although the health risk of taking in other's breast milk is not high, do consider these Counter-arguments if you are on the accepting side.
Make your own Breast Milk Jewellery
Turn Liquid Gold into Solid Gold. Keep it for yourself.
Or some day, your son could use it as betrothal gift.