the tiredness is an extremely common symptoms of depression- infact i beliee its one of the symptoms used to diagnose it- that and "low mood" are the two big diagnosis points.
I used to be like you- i could sleep for 16-18 hours a day and still be half-asleep/ too tired to do anything. i used to find it physically exhausting to even go to the shops 2 minutes down the road- sometimes even a SHOWER was too hard, i'd just skip it.
i find its a combination of many things- the RIGHT medication (there is more than one of course, and if the one you're on isn't helping than speak to your GP about another one), therapy and learning coping skills/ new behaviours, lifestyle changes. for example, i found i got into a "rut"- every day was just me sleeping as much as possible, playing WoW and talking on Vent whenever i was awake. i lived on microwave meals and noodles as i was too tired to cook, and was only able to work part time (in a job i hated).
The medication really took the edge off- it regulated the sleep better, so i'd get a really nice 8-9 hours then wake up refreshed and energetic enough to actually do stuff. it does take awhile to build yourself back up- you wont go from no energy to heaps in one day. but once a med starts to work, everything is just the little bit easier, you're just that little bit less tired and less unhappy. So thats when you have the energy to do the rest of the stuff- the therapy, the lifestyle changes, the healthy exercise (also get some sunlight each day- very important).
Each day i slowly got my energy back and now i'm probably more energetic than most people- i can get a lot of stuff done and i can multitask really well. i'm now studying my dream course for my dream job, i have enough energy to do crossfit everyday!, i quit playing WoW (just because i have so many fun things in the real world to do, i didnt have time for it- but you can still make time for it if its important to you). i cook healthy meals each day, clean the house and do the chores, do volunteer work (am still trying to find a job), do lots of errands, and have lots of fun hobbies- now it seems like theres not enough hours in the day to do everything i want to do! infact i had to learn to slow down a little bit as i was starting to get exhausted from doing TOO much- now i'm better at making time to just relax and do nothing.
so yeah, seek out a good therapist, and ask your doctor again to review your medication. a different med might fix your energy and mood!