Agree with what Kizzie has said, but it isn't just you that is struggling like this bm. I am trying to recover from a severe depressive episode and was in hospital for 3 months earlier this year. My last episode was 15 years ago and I also spent 3 months in hopsital but I was lucky then,because my recovry was straightforward.
I know exactly what you mean about scrambling up the hill and just as you think you are about to reach the top you slip down again. I don't think you slip right to the bottom to be honest (though I know it feels like this) but I try to look at it now as getting stuck on a rock on the path, or slipping off the path when a run of bad days come.
I have always kept a journal and I rate each day - Awful, not good, not good/not bad, reasonably ok, good, very good. Recently I thought of adding up the totals and was amazed to find I hadn't had nearly as many bad days as I thought and that alone pushed me up the hill a bit further. But like you I never know when I am going to slip again and am not really able to make arrangements cus never know how I am from one day to the next. This I think can be worse than being down all the time, becasuse your hopes get built up only to come crashing in on you.
I have a really good CPN who visits once a week and has been a big help to me and has being doing CBT with me. Have your tried this at all. It's not a quick fix but it can help sometimes.
You are managing to hold down a job so you obviously are very strong in spite of the depression. You say you have had m/h problems for years - do you know the cause of these problems. Whether you do or not I think that therapy would help to unravel the past andmaybe gain a better understanding of it.
Also don't be worried about upping the meds - whatever helps I say - do it. I had a friend who suffered for 12 months with drug res. depression until a new psychiatrist added lithium to her ADS and that really was a turning point for her.
Have you got supportive family/ friends/H/P who you can talk to as I have found this very useful.
Sending best wishes and hoping there will be brighter times ahead which I'm sure there will be. Depression is a deceitful illness and leaves you feeling worthless etc BUT the vast majority of people DO get better so hold on to that, though I know how hard it is to do that when feeling so awful. Trouble is there are no words to describe the awfulness of this illness and everyone's depression is different and treatments that work on one person, are no good for another, just to add to the awfulness of it all.
I read somehwere "the waves of depression will come but you can learn to surf"