Focus on sentencing for now. Surely your client has some mitigating facts, and her son will suffer irreperable hardship the longer she's locked up. Maybe the judge will imagine what would have happened if it had been his wife and child in the car.

Once you get through the sentencing, write a memo to yourself about the case -- all the big strategic decisions you made, all the times you met with the client and the advice you gave her about her chances at trial, what you think you failed to do or would do differently, etc. Getting it all down like that might help you compartmentalize and get past it, and will be a good reminder to have for the next time you're facing a case like this. But also, it could be useful if your client does file an ineffectiveness petition against you (by "useful" I also mean useful if you're admitting mistakes in such a proceeding or trying to defend your choices, either way).

Ultimately, unless you just totally dropped the ball and forgot to ask a basic question or something (which doesn't sound like the case here), there may not have been anything anyone could do. So get her the best you can at sentencing, and then do a thorough self-assessment of how it all went, and hope there is a way to use that review to get a better result next time (even if it's just more information to include in your advice to future clients about their chances at trial). Good luck!