As a simple negative of happy, the adjective unhappy has a fairly broad range of application. A person described as unhappy may be sad or miserable, or they may simply be displeased or discontented with one particular thing, as when one is unhappy with the outcome of something. The word miserable, on the other hand, leaves less room for interpretation. A miserable person is intensely unhappy, to a pitiable degree. Where miserable comes in especially useful is in describing a wretched state or condition, such as a miserable cold, a miserable pregnancy, miserable weather, miserable living conditions, or, worst of all, a miserable existence. Miserable is a more feeling word than unhappy, and carries a strong element of the pathetic. As with all things pathetic or wretched, it can evoke either pity or contempt. For example, there’s no empathy in calling someone “a miserable so and so”: for miserable here you could just as well substitute lowdown or despicable.
Something that is excellent is remarkably good. The synonym sterling means “thoroughly excellent,” but its history adds a little shine. Sterling entered English as a noun referring to the silver penny of the Norman dynasty. Over time, it came to be used to refer to British money more generally, as in "pound sterling." Along the way, sterling developed several attributive senses relating to money and more specifically, to the quality of metal used in money. This is where we get the phrase “sterling silver,” designating silver that has a fineness of 0.925, now used especially in the manufacture of table utensils and jewelry. The use of sterling as an indicator of quality was extended to things immaterial, such as a sterling reputation—to indicate that they meet or exceed the highest standard.
To lessen something is to make it less, or to reduce it. If you hope to lessen the chances of something bad happening, you want to make the number of chances fewer. Mitigate is a strong synonym for lessen, but this verb is used especially to talk about making things more bearable or giving relief, as to someone in pain or sorrow. It’s also used to talk about making things less severe: to mitigate the effects of climate change. Mitigate is sometimes confused with the similar sounding verb militate, which means “to have effect or influence.” This mix-up often occurs in the use of the phrase mitigate against, e.g., This criticism in no way mitigates (read militates) against your going ahead with your research. Although this use of mitigate occasionally occurs in edited writing, it is rare and is widely regarded as an error.