A finite additive measure that is continuous from below is a measure

A finite additive measure is a measure iff it is continuous from above. [sorry, I made a mistake in the title, should be "from above"]

I finished the proof from finite additive to continuous from above, but stuck on the other one...

Proof so far.

Suppose that is continuous from above, that is, if with , then I have .

I will show that is a measure.

Let be disjoint sets.

Claim:

Let , note that and

Now, since is continuous from above, I have

I'm defining the sequence as such base on how I work the other direction, but I can't seem to be able to break the left hand side into a sum of .

Any hints? Thank you