Why am I always tired?

After a trip to the doctor, I changed my diet and lifestyle habits. It's been 3 months of healthy snacking all day with healthy proteins and cutting out sugars that give you momentary "highs" I do pilates and walk everyday and get at least 7-8 hours of sleep a night, but I am still on the brink of falling asleep most of the time unless I'm really engaged in something. What could be wrong?