Sorry to hear about your depression Adam, hope you are getting some support ? Unfortunately being ill in itself can heighten the mood, so if already low, it can worsen things.
It's luck of the draw sadly, some can have good resistance to colds/viruses, others don't, but as mentioned before just taking more steps to help prevent them occurring will help, so keep hand sanitiser on you, boost your immune system, echinacea seems to rate highly in helping with boosting your immune system along with Vit c, also try to get out for fresh air/daylight/exercise if you can to help boost your immune system during the winter months.
I get the odd cold but nothing different to when I was a non t1.