muma3 - It's ok to feel bad right now. What's just happened is very hurtful for you, you will feel you need to protect yourself.
Raising a family is a career, a valid and useful contribution to society. As you have had your girls when you are young, you will have plenty of time for another career when they are older, if you want it. You could perhaps look into working, studying or voluntary work part time when your littlest is a bit older.
You will feel like the bottom has fallen out of your life right now. I can remember driving along the motorway, screaming out loud in my car because I felt so full of unimaginable pain that I was going to burst. It had to come out somehow.
When dd was tiny, I felt so angry and worried that her father had damaged my enjoyment of her early milestones and left her with a mummy who was only half there for her emotionally. I couldn't help feeling depressed but I was angry at him for her sake. I was worried that I wasn't doing a good enough job for her but she is ok, she is fine and things are a lot better now.
You are a good mum. I can tell this because you are thinking of them and you mention Brownies, talent contests, potty training. Your girls sound outgoing and creative. I'm sure the thing they want most is to please you and to have you appreciate their achievements. It is important that you get better asap for your children but don't beat yourself up, it will take time, they will be ok as long as they have you.
If you find that you are really not managing (emotionally and/or practically) and you can't get enough help or support from friends and relatives for whatever reason, don't hesitate to go to the doctor. Sometimes when you are incapacitated due to emotional problems, all the practical problems pile up and problems are compounded and take much longer to sort out. It's important that you don't let things get out of hand and overwhelm you. You don't have to take antidepressants forever but they might be enough to get you through a bad patch in the short term. Or otherwise, try natural remedies like St. John's Wort, Bach Rescue Remedy, Valerian or Kalms tablets for anxiety. Eat properly. If you are tempted to drown your sorrows, try not to drink too much as it can easily become a habit that makes you depressed.
We know that when men make a decision to end a relationship, they generally seem to find it easier to switch their feelings off and move on with their lives. It doesn't mean that he didn't care about you but it does sound as if it is definitely over. Keep telling yourself that it is better to have loved and lost than never to have loved at all. You've just been unlucky. Don't be grateful for a man who is willing to take on you and your kids. Be extra fussy about who you let into your wonderful family. The key is that you don't really need a man. Life is often a lot easier, and sometimes more fun, without one!
You are grieving. Unfortunately it won't get easier in the short term and it might get worse. Go easy on yourself. Be kind to yourself. Look after yourself and your kids. Just endure for the time being.
You know that there is no easy fix to this but keep posting if it helps.