Anyone experienced "plateauing" with anti-depressants?

So I'm a life-long dysthymia sufferer. It sucks. For the last two years or so I've been on Effexor XR 75 mg, and it has been really great. I finally learned what it felt like to NOT feel awful all the time. I stopped having intrusive negative thoughts, stopped obsessively dwelling on small set-backs, stopped feeling like strangers were always staring at me, stopped having suicidal thoughts, etc.

Within the past week or so, I suddenly feel like I did before the medication. One day I woke up and felt like I had what I always called "brain fog"...a really cloudy, unclear feeling in my head. it lasted all day and felt terrible. All of these awful thoughts have returned...I can't stop thinking how bad and fat and stupid I am. Everything can be fine and I'll think that I wish I was dead out of nowhere.

So have my meds worn off? Has this happened to anyone else? Will the dr up my dosage? What happens if we have to keep upping the dose until it maxes out? I'm stressing.