If you absolutely cannot take time off, and you absolutely must work then you must change something about the thoughts that are making you sad. Its easy to say stop thinking about it but the crux is that you have to learn to stop thinking.
Every time it comes to your head; imagine a big stop sign; and say in your head and quietly to yourself out loud (important for all 3 as its stronger using your actual voice than not) "not now Josephine" (in the ilk of Napoleon) or some other suitable name? someone you dont like.
Imagine the "bad" thoughts being said in the voice of someone you dont like - some pompous self absorbed eejit (enough so that you feel well able to say - no! go away). The thoughts will return. Just keep doing it, sometimes the thoughts will return immediately, sometimes you might make yourself smile and they go away for 5 minutes, but keep doing it, change the voice. change the statement, do whatever it takes and then immediately go on to think about something else, whatever you can, whats for dinner?, which client is next? - it doesnt matter, as long as its not sadness.
At the same time place a hand on your chest and say to yourself " I'm safe, this sadness will pass". You're trying to rewrite neural pathways that are now predisposed toward sadness, when the sadness is serving you no purpose.
You're the one crying, you're the one sad, its changing nothing. All its doing is making you unable to function. Sadness is not serving you.
I'm not saying the sadness isn't valid. I'm not saying that you should never let the sadness out. I'm saying that you have to find coping mechanisms to get through those 5 minutes. And this is what worked for me.
Eventually the sad feeling will not come back immediately, some days they will, and thats ok. But you are trying to get through that immediate 5 minutes. And the next time, its just the next 5 minutes. Until one day itll be 10, then 15.