Hollywood has a lot to answer for. The way I see it, none of us has a right (nor should we expect) to be 100% happy 100% of the time. After all, how can we know just how good the good bits are if we don't have boring and/or crappy bits to compare them to?
I'm sure you have done all the stuff people have suggested - looking for people in a worse state than yourself etc etc - but there is a difference between UNDERSTANDING that you are in a better place than most and actually FEELING it.
Decide if you could be clinically depressed or not - the use of the words "loss and sadness" suggest this is more than being a bit down in the dumps. If so, go to your GP for some help in sorting it out.
If not, take matters into your own hands. You CAN change where you are, professionally, in your relationship etc. Ask yourself what is holding you back - is it money, or lack of support, or is your self esteem slipping? Have you talked your feelings through with your partner? Do you have good friends who can help you offload over a bottle of wine or a cup of tea?
Do you take time to enjoy your kids, rather than just caring for them? Mine are now in their 20s and when I look back at their childhoods I see too many times where I bothered about balanced diets, regular bedtimes, homework and their petty arguments and not enough times where I just sat and appreciated them as funny, clever, loving little souls. Older people always tell you to enjoy your kids, and I remember thinking "Oh sod off - how can I enjoy lack of sleep, teething, lack of social life etc??" but you know what, they were right!
Anyway, the good news is that it WILL get better. I thought my life was over at 30 when I had three kids and no career, even though DH had a well-paid job and we had a lovely home. But it was a passing phase (albeit quite a long one, five years at least!). By the time I was approaching 40 I began to feel more in control, more empowered, went back to university and started down a career route. Kids became teenagers and much more capable of looking after themselves. DH struggled with me not being a stay at home mum at first, but eventually got over it and even learnt to cook.
And now I am in my 50s and despite wrinkles and menopause, life really is the best it has ever been. My kids are wonderful independent adults, DH and I are like a young couple again, and I have a career I love. Life IS boring sometimes, but no pain, no gain ... by the time you are my age, you could well be looking back and wondering what you ever worried about. Just keep thinking positive and remember, the only thing that can stop you achieving the life you really want - is you. Good luck, lots of love xx