Well I'm no expert or dietitian but I think the general advice is to eat complex carbohydrates rather than refined ones (things like wholemeal pasta, brown rice), grains, oats, seeds, nuts, beans and so on...
If you google 'slow energy release foods' you should get some good tips.
If you really are feeling so lethargic, and you're sure that you're eating enough calories, then maybe you should have a trip to the doctors to check that nothing else is going on.
It seems unusual to suddenly start feeling so tired and lacking in energy after taking the Xenical for so long, especially if you haven't particularly changed your diet or exercise regime recently.
Let us know how you go on x