Oh Lynda - so so sorry you are feeling so awful. I understand exactly how you are feeling because I have had 2 major episodes of depression (the last one last Easter) and am still not fully recovered. I have good days than a setback/blip comes along and I'm back feeling flat, hopeless, yes world's worst mother, partner, friend - I'm not worth bothering about etc etc.
You say you have suffered depression on and off for many years - did it begin in teenage. Have you any idea what caused the illness. Have you ever been on medication. Sorry to bombard you with questions - just trying to gain a bit more understanding of your situation.
Your first port of call must be your GP - and get on medication unless you are already on it - if so, maybe the dose needs to be increased, or the meds changed. Lynda you haven't "finally lost it" - it's just that your depression is worsening as you say and this is very very scarey, especially as you have little ones to care for. Do you have a supportive DH/DP family or friends in whom you can confide.
Please don't apologise Lynda - we all vent our feelings on this MH thread and there is loads of support.
You may need some therapy (especially if your depression was caused by some childhood trauma that has been buried, so to speak). The GP can refer you, but usually its for CBT (cognitive behaviour therapy) which doesn't look into the past but helps us with "the hear and now" - it isn't a magic bullet but it an help.
Depression is a horrid horrid illness and it is deceiful because it makes us believe things about ourselves that aren't true and makes us think we "don't want to be here" not in the sense we want to die, but we want the awful scarey feelings to stop. In the vast majority of cases people recover fully from depression. You must believe that it won't always be like this Lynda.
Look forward to hearing from you and in the meantime sending warm wishes. There will be others along soon as there are many of us MNs suffering from depression, and mostly like you, with young children to care for. All mine are grown with families of their own.
Take good care and don't believe the thoughts that you are having are facts - because they aren't. It's the depression telling you lies.