Hi Curleous,
What you're going through sounds very similar to what I went through earlier this year. I was mainly depressed and feeling down most of the time, but also got snappy and irritable, especially with my wife. I'm not perfect, and still have the odd 'Grrrr' moment, but I've got to say that addressing it was the biggest factor in the equation - possibly verbalising it cemented what I was feeling inside and forced me to do something about it on the outside.
I brought up the subject of lantus with my diabetes consultant and he dismissed that as a first port of call. He talked about blood sugar levels and other factors in my life. We discussed the chemical nature of depression (as opposed to the purely psychological side). When he got to the point of prescribing anti depressants I decided to give it a go without and see how I got on.
I'm convinced it was purely admitting to a professional how I was feeling and talking about my feelings and other circumstances (work, personal life, exercise, etc) that brought me out of that spell of feeling down and facilitated my doing something about it. That and the responses I got from the other forum users.
Have a think about something you may be doing that could be subconsciously making you feel down but isn't the obvious answer. For example: I know someone who had spent time at work flirting with a colleague, which indirectly made him go home and behave horribly to his loving spouse. He didn't realise this was making him behave in that way, and it was only when he admitted it to her and got over it that their homelife returned to normal. I'm not saying you are like this, but the mind is a curious thing and can often do things for strange reasons.
Or it could be the chemical imbalance that my doctor talked about.
Whatever it is, it's good to talk to specialists and rule things out. Get things off your chest when you talk to your doctor later this week and things might sort themselves out!
Let us know how you get on,
Sam.