It's the worst time of year for messages like that, often people are stuck in stating the same message without thought to adapt it as they are sometimes robotically working their way through preparations as expected by the masses. And often, they'll want to send a card to show they care but don't realise the message printed inside jarrs so much with how we feel. Try and take a deep breath and think of the care they have for you beneath it. If they haven't lost anyone themselves unfortunately they can't grasp the full extent and depths of the emotions
Are you able to take some time to have a nice bath or shower to start with? Something that feels a little pampering (whether that's just because its not so rushed). It's totally normal to feel so bad, all I can suggest is treat yourself as you would a friend - understanding and gentle.
Sometimes I found a dvd/duvet day really helped me - giving myself permission to stop for the day and do something indulgent. Gave my mind a bit of respite. As long as we don't allow ourselves to drift into doing that everyday it can make such a difference. Permission to stop, be, feel sad without pressure on ourselves to get on with things xx