how can i lose weight effectively

i lost 3 stones years back after acquiring diabetes type 2, so i needed to lose the weight, i managed to get quite slim....but now even though i'm still doing well managing the diabetes, the weight has crept back on around my stomach and waist....i'm anxious to burn the weight off...i go swimming 3 times a week....thats as much exercise as i'm capable of..

i'm now 44 years old, what can i do?