Grief is horrific and IME the three worst times in grief are immediately after the death (where you are now - in shock and raw pain), after the funeral (when often support magically disappears) and that point where you realise 'this is it forever' (which can be different for different people but is often in a range of 1 - 3 years). You are are in the darkest worst time right now so it's normal to feel the way you feel.
Take it a day at a time, even a hour at a time if that is what you need.
Do what you can in your circumstances to get a break and some space. If you are under pressure to perform anything (childcare, work) do what you can to get a break. If you are trying to work, stop and have a break. If you have childcare responsibilities, ask your partner or non-bereaved relatives or friends to help out for a few days.
Do speak to your GP. Things like diazepam or sleeping tablets can help get you through the first weeks. DO NOT take antidepressants at this stage. You are not depressed you are in extreme grief. This is not depression at this point. It is extreme emotional pain at loss.
Breathing exercises can help. Key is to make out breath longer. Breath in for four count. Hold breath for four count. Breath out for eight count.
Immediate change of your environment can help too - so if you are cold, put on a warm jumper and have a hot cup of tea; if you are warm, go outside if it's cold or suck an ice cube; go for a walk. Anything that makes a 'jolt' to your environment. It only needs to be short but it can help to knock you emotionally out of the worst anxiety.
Those are my starting point advice thoughts but I am sorry for your loss as grief is so evil and a sudden unexpected death adds to natural shock.
Long term grief counseling can help but you need to be at least 3 months post now because when its too raw you can't discuss it and have no perspective. Do bank that as a long term plan though.
Finally come and hang out on the Bereavement board here - it's in Body & Soul section. There's much good advice there.
Condolences to you.