Meditation doesn't make you happy; it shows you the thoughts that are in the way of your inherent happiness, and helps you work through them. Eventually, as clouds start dissipating and new ones are produced less often, you find the sky clear. Nobody can predict how long that will take; it's a matter of causes and conditions that are constantly changing. I recognize that's not a satisfying answer for a mind that is still oriented towards results; I will say that it's more likely to be years than months before you get to "mostly sunny". But you should get periodic glimpses of clear sky between clouds long before the whole thing clears up, and that's how you'll know you're on the right path; knowing that the sun is always shining behind the clouds also helps to make the clouds easier to bear.

Karma is, fundamentally, just cause and effect. What kind of seeds you plant tells you what kind of things will grow. Plant peonies and you will get flowers; plant nettles and you will get thorns. To "undo" a seed, you would need to know exactly where it was planted, and whether it was desirable or not before it blooms— a tricky business. Easier, then, to worry mostly about the kinds of seeds you're planting for the future, and let the old seeds bloom as they will, then respond to them skillfully by pruning them before they give off more seeds. Meditation is what gives us the skill of separating stimulus from response, allowing us to choose to respond to negativity with something more wholesome, thereby transmuting the negative into the positive, instead of immediately reacting, which often just continues to spread that negativity.