complete lethargy/lack of energy a 22 year old should not be

Hi, Im really hoping somebody out there can help me, as Im getting desparate!
Im a 22 year-old female from Costa Rica and have been suffering with an unknown illness for several years, but it seems to be getting worse and I cant keep going like this! I have been to many different doctors, psychiatrists etc here in Costa Rica, but none of them have been able to help me so far... so Im reaching out to the rest of the world for solutions.
My symptoms include regular severe headaches and complete lethargy/lack of energy - not something a 22 year-old should be facing on a daily basis. It means I find it very hard to do anything active and just feel I want to sleep almost all the time. Also rapid mood changes are frequent. As I said, I have visited pschiatrists who suggested it may be depression or some kind of mental illness, so I have been taking powerful drugs including Lamictal as well as prozac. After discovering that these didnt seem to help at all, the doctors put me on a stronger dose (3 prozac + 2 Lamictal per day). This has also had no positive effect.
I have had several blood tests to see if there is an internal problem, but as far as the doctors have found, everything seems to be in order.
If anyone has ANY idea what the problem could be, PLEASE get in touch as soon as possible. I will be eternally grateful to anybody who can help me.
Thank you very much,
Dani