In this case "as" is a preposition and "losing" is a gerund.
Here are examples of "as" used as an adverb in comparisons.
He lost that match as easily as he had won the previous one.
It was as bright as day.
While it's technically an adverb when it modifies an adjective, you can also think of it as a required particle in a comparison. The as..as form of a comparison is one important type of comparison.
Usually you would have to have 'as' twice to make the comparison. In the example you gave, no comparison is being made.
In the example you gave, the gerund functions as a noun.
You are losing.
An example with 'as' used as a preposition with a noun instead of a gerund is:
He wanted to be seen as a rebel.
He wanted to be seen as himself.
It came as a shock.
And here's an example of 'as' functioning as a conjunction:
I watched as he came forward.
"I watched" and "he came forward" are independent clauses. For various meanings you could substitute other conjunctions such as "and", "but", "while", or "because". In this example "while" would be a synonym.
In the following example, "because" would be a synonym.
I watched carefully as I had never seen someone do it before.
It's only functioning as a conjunction if you have two independent clauses. In other words, they can be a complete sentence on their own.
I watched carefully. I had never seen someone do it before.
In the example you gave, 'as' does not join two independent clauses.