Why does reducing weight after 40 become so hard?

i'm a 41 year old mom of 16 years old twin sons.... i was always naturally slim even till age 38 and used to enjoy the refreshin remarks about me lookin young and not at all like a mom of teenage kids....... but now i've developed some fat around ma waist (equivalent to a motorbikes tyre) have been swimming all summer (around 45-50 mins of swimming daily) but my weight didn't come down even a single point ....... i am around 66 kgs with height 5 ft 6 inches ........ i confess that i can't diet ...... but at the same time i'm not a person who eats alot.......... suggestions plzzzzzzz to look smart and reduce atleast 5-6 kgs of weight