why am i always tired?!

hi guys just wondering if any one is or has experienced what im going through?
im almost 32 and have 3 children and i am completely wiped out all the time, i cant blame it on being busy because of the kids cuz if i have a very easy day then im just as tired.
it all started after i gave birth to my 1st born 11years ago, when she was a few months old i started taking naps cuz i became extremely tired, how i don't know cuz she was a really good baby but the tiredness has become a lot worse now.

i cant go through 1 single day without going to bed and i feel ashamed to admit this. been to my doctors a few times cuz i cant go through life like this and the 1st time he said i was depressed which i disagreed an was sent home with meds. so years later i go back and they run bloods test for anemia an thyroids which both come back negative,im going to go back and demand more tests cuz i just cant live like this it is ruing my life and my children are missing out cuz mums always to tired to do anything with them, today's routean went like this, woke 8.30 got the children breakfast then dressed, done a few chores,make beds an clean up after breakfast, wile children were doing arts and crafts i done bout an hour and a half of ironing then made lunch. after lunch we had a walk down to the park and was out for half an hour before i had to come home to bed for a nap. slept from 2pm till 3.30 pm and now its just after 10pm and i am knacked. this is normally how my day goes and if i don't have a nap in the day then i am in bed by 8pm with nothing done.
i have a really supportive hubby and he just seems to think i do to much but i know deep down this napping is not normal. i even had to finish college cuz i was falling asleep during lessons and couldn't do anything with the children when i got home cuz i was that tired.has anybody been through this an got a diagnosis from there doctor. any help is appreciated