Hi @Shorelle,
Big hug. <3 Getting enough rest would be a magic trick in your situation. But if there's some way you can wangle that - more rest will help you regain not only physical, but some emotional energy too. I hope you have some options there - like occasional babysitting or the girls having a sleepover with someone you trust etc.
Whether or not that's the case, your surgeon, oncologist or GP could refer you to a clinical psychologist. I don't think any of us should try to get thru this without that level of professional emotional support. I and a colleague who also has bc are both seeing psychs. Wouldn't be without it. A psych will also help with some practical tips for easing your responsibilities too.
I'll go out on a limb here and guess that despite your diagnosis - you still have too much on your plate. Your lovely daughters will need you to be more 'selfish' for a while - they will thank you in the long run. I get that fear you have. My daughter really only has me (except some clueless relatives who's values make me cringe). So we just have to keep believing that these girls will have us 'bossy mums' around for years to come!
If the fear starts sucking you down into that 'sh*!t - scared' horrible place - try to breath deep and remember whatever stats and facts are in your favour (then let go of the rest) - it seems to help me. My surgeon is really good at reassuring me when I'm having a mini-freak-out. I hope you have someone - or find someone who can do that for you!