What is wrong with my diet?

I have been an on and off dieter for many years. I have lost a total of 20 kilos in all and am just now trying to lose the final 5.

My diet goes something like this:

Breakfast: 2 eggs poached or scrambled occassionally with 1 slice of low-cal wholemeal bread

Lunch: Tin of tomato flavoured tuna with a small tub of tabouli

Snack: Another tin of tuna

Dinner: Some sort of meat or poultry (fish, steak or grilled chicken) with veges or green salad

I go to the gym 5 days a week for an hour and a half combining weights and cardio in the mid-morning. Usually I walk 5km on the treadmill, 20 mins on the cross trainer and half an hour of weights.

I have been doing this for 3 weeks and have lost half a kilo. Is there any reason for the slow weight loss? It is disheartening. Can anyone see anything wrong with this diet?