Wish I was happier and a better mother

[iwishuponastarr]

I have 3 DC who I love dearly but I'm really struggling with DS2 behaviour at the moment. He's 6 years old and I don't know what to do anymore. He can be the sweetest, funniest, charming, sociable, wise little boy but he can also be stubborn, rude, mean to DC3, answers back, isn't respectful etc!
I try my hardest to be calm and to try different approaches but nothing seems to work!
I find myself declining invitations to places in case the two older ones kick off with each other. I always seem to be moaning at them. I try so hard to explain to them how they should behave, to be respectful, to be kind...
I don't know, I just feel like such a failure.
I was always maternal but having them has made me think I'm not as maternal as I always thought I would be.
I worry about the future and how they will turn out. I worry what other people think of them (and me). I just feel low a lot of the time and sad that this isn't the life I thought I would have.
DH is supportive but home late during the week so everything falls on me. He does take over a lot during the weekend, but I feel like I'm passing the baton to him when I really want to be a team.
My fear is that they'll look back at their childhood and think they had a miserable mum who was always telling them off or that they never felt good enough. I want to feel that they are good enough but their behaviour sometimes makes me feel otherwise.