I think you just have to decide to be happy. As Lincoln said, "Most folks are about as happy as they make up their minds to be."
I know this sounds a lot simpler in theory than it is in practice, and I'm aware that I may be coming across as some completely annoying Pollyanna type who deserves to have darts thrown at her image. 
But bear with me
How many times have you (not you specifically, any parent really, me included) found yourself saying "no" to your DC out of habit because you're feeling a little irritable, rather than for any good reason. And how much nicer would it have been if you'd said yes (assuming their request was reasonable, of course)?
How many times have you spilled tea and thought FFS!!!! and been put in a bad mood for the rest of the day when instead it's just a bit of liquid that can be mopped up with no harm done and can be forgotten in 5 minutes.
Negativity breeds negativity, and positivity breeds positivity. It's self-perpetuating and habit forming. The more positive you try to be, the easier it becomes and the happier you feel.
Here's the serious bit - this only really works if you're happy with life and the bits you have control over in general. If you're married to a wanker or your boss treats you like shit for example, it probably won't work. It might if you ditch the wanker and stand up to your boss or get a new job though.
Stress (or more precisely the inability to do anything about what's causing it) is the biggest enemy of feeling happy. To that end, try to start off on a de-stressful note. Having less sleep and not fighting the clock in the mornings will make you far less stressed than that extra 20 minutes in bed. Although sleep is important. Diet and exercise are too. A healthy body is well-equipped to process all that cortisol in a much better way than an unhealthy one.
Good luck. 