ssri's and breakthrough depression

So I've been taking 50mg of zoloft for 4 months now and unfortunately I'm getting depressed again and have been feeling really low, unable to eat, and back to feeling like I'm in the hole and won't get bette again. My depression was pretty bad for me back in Oct and I started with Lexapro and eventually switched to Zoloft 3 months later due to constant headaches. Zoloft worked and I've felt pretty good for a while. I'm not sure what happened to make me start to spiral downhill again. I was hoping 50mg would be something I could consistently stay on for a year and go from there...is what is happening normal and what causes breakthrough episodes to happen on meds?

I did...I'm supposed to wait a few more days and try to ride this out...it's not fun since it's already been 4 days. Does an increase in dosage normally help in a situation like this? I fear this will keep happening over and over until I max out on the drug in time.

Thanks Karen...I appreciate your reply, they always make me feel a little better. I know everyone is different in terms of treatment - depression is a complicated thing and next to impossible to try and explain to someone who doesn't have experience with it or hasn't dealt with someone close to them live it. I envy those who don't have to struggle through life with mental illness or don't always feel that this beast lurks around every corner, making you terrified of slipping into that awful place again. At least it is treatable for the most part though - I know many other people have it way worse out there but it's hard to keep that in the forefront of my mind when I'm not feeling well and trying my best just to put one foot in front of the other some days.

As per my doc, I've given myself a full week to try and pull myself out of this funk. I just can't seem to do it and the more days I wait the worse I slowly am becoming. I thought I might be able to do this but my appetite has lessened considerably, I'm back to not sleeping and i just feel like I'm walking around with weights on. It's strange I can even feel this in my hands and feet-I just feel physically and mentally off. Im disappointed and feel I should of been able to do this but I can't - when my sleep and appetite starts to go that isn't a good sign. I start to wonder if there is something else wrong with me when this happens...the nature of the beast I guess.