If you can pinpoint a reason behind the depression then meds for a short period to give you time to resolve things are understandable. But some doctors are too slaphappy with the pills.
It's not that you need to be stronger. It's that you need to sort out your boundaries. YOU deside it's over and he's never coming back. Then get a solicitor deal with everything else and focus on getting healthy again. People can't yoyo back and forth if you don't let them. And don't use the kids as an excuse because your children deserve a happy, healthy mum most of all.
Stop taking on his baggage, outsource it to a solicitor. And get yourself on a nice long, sunny holiday if you can afford to do so. Or a spa/countryside weekend away without the kids or something.
You don't need to be strong. You need to be kind to yourself. And learn to say no to people who are looking to take the piss.
I'd go back to the doctor. Ask if you can put a pin in the valium for now and focus on 'me time' instead? Might be worthwhile to take up running or yoga, something to help release endorphins or quiet the mind. Then once you feel calmer in yourself, see the gp again about coming off the depression meds.
Your aim should be to get better first and foremost and second, to get off the pills. Staying on them this long may be creating dependency.
Focus on making the changes to your life that will make you happy.