How hard it is to stay upbeat when your spouse comes home a grouch?

It depends: It depends on how resilient you are, and how grouchy your spouse is! if s/he is more often grouchy than not, and also unwilling to own this, the difficulty escalates. The more positive emotional supplies you have to draw on when things are tough, the more you'll be able to deal with the inevitable ups & downs of relationship. If your spouse is absolutely your only support, not healthy for either.