Hi there, I am not a doctor but I can give you my opinion based on my own experience and what I have learn about it.
First, I think there are two distinctive kind of depression one been a symptom of some dramatic expirence ( like the dead of some one realy close, etc ).
The second one ( the worst ) is a quimical unbalance in your brain, this kind you can also subdivide into two diferent once, bi-polar which basicly means that you have realy low depressive periods but you also have periods in which you are realy hipper, exited and can do any thing that you want.
The other one ( the one that is beating me up ) is sever depression.
I have been diagnost with this about three years ago. I do not question way anymore, at first it drove me crazy and I wanted gone yesterday but unfortunaly for me I haven yet foun a medication that works.
I have tried everything, some times it works for a month and then stops and I have to try another med.
I am a vegetarian ( for the last 30 years ), I am an alcoholic ( I quit drinking when this start 3 years ago ), I never give a deam about exersise or take care of my self.
It could be any of these things, I do not know but it does not matter for me anyway, what matters now is to find a med that give my life back.
There are periods that I can not move from my bed for a month, do not take a shower for weeks, it gets realy bad in my case.
I found out that alcohol is bad for depression so I quit, coffee (specially) whith sugar is also bad for depression ( I use to drink two pots a day ) I aslso quit that, nicotine is also bad for depression but I smoke since I was 8 years old, I do not have it in me to quit that.
I check out the natural stuff and after talking to my psycaystry ( regular doctors are not the best for this problems so make sure to see a phsycastry ) I am taking B12 vitamins, Fish oil capsuls, Gingko Bilova, and my prescription Zoloft.
For the last month or so I have been feeling better, as unbelibeble as it may soun what did the tric for me was to change my scheduld, I go to sleep at 3:00 pm and I wake up at midnigth, play in the compu until 7:00 am take a shower and go to work until 2:00 pm and at 3:00 pm I go to bed and start the cicle all over again.
There is a study that shows that sleep depravation makes yor brain produce serotin which happen to be what the meds try to do.
In respect to been tired and dizzy it may be that the medicine you take does not agree whit you, however it is true that depression can beat you up and make you realy tired.
It may also be your thyroid, have it check, it have similar symptoms.
Good luck, I hope you get something out of this.