its not your fault and you have not let anyone down -imagine if you went to work and because you cant posibly be expected to concentrate made an error -which had catastrophic effects. Your colleague could have simply been trying to make some kind of general chat -rather than trying to make you fel guilty -maybe they were trying to make you feel wanted and needed, but if their motives are selfish they wouldnt be texting you if you had a physical injury they cannot possibly understand what is going on unless this has happened too them -and even then everyone reacts differently.
You havnt let your husband down either -we are not some kind of automaton that just carries on regardless -even though society sometimes would prefer us to be, but your husband would understand that you are not a robot -if the situation was reversed would you expect him too have been. But guilt is also part of grief and we will look for anything we can find to feel guilty about.
I went back too work too early i made a couple of mistakes sent the wrong invoices too the wrong customers - Fortunately my boss is a human and i didnt get into trouble. When i wasnt making mistakes i was in a corner crying some-where no use to anyone.
One thing others have done is to arrange a phased return to work -maybe your company would consider that -so you dont get thrown in at the deep end when you do return.
I also understand your not wanting to go back -part of it is that guilt thing -because going back sugests a return to "normal" and yet how can anything be normal ever again.
Part of it is not wanting to face colleagues -how will they re-act will their sympathy leave you a sobbing mess - will their lack of it do the same. One of my bosses although not without sympathy was telling me all about an argument at home with his wife about the colour of their kitchen units - like i gave dam - i wanted to scream at him -lifes too short let her chose she might be gone tomorow -but i didnt -somehow you learn to bite your tongue - and i know he was a young lad -never lost anyone - but at least trying to make conversation despite feeling he didnt know what to say.
Also there is a feeling of how pointless it all is -here we are slaving away to put a roof over our heads when under that roof is a house full of empty rooms where the most deafening sound is the silence. In my case my whole job is pretty pointless Who cares if some-one got the wrong invoice or an advert doesnt get done on time -its all trivial compared to this loss.
In its favour, however, work does at least give your day structure -you have to get up,get dressed,clean your teeth, brush your hair, and eat in order to do it -and often we dont want to do that and when off sick dont have too -but if we have to work we do - Routine is dull and boring, but routine also helps us deal with the things that have happened which are completely the opposite of it. its a small step forward but small steps are the only ones possible.