Could use some advice on keeping finished attic cool in the summer

I'll start this by saying that while I consider myself pretty handy, I know there is so much to know about HVAC that I think you guys and gals might be a good place to consult. I live in a home built in 1910 that the upstairs is a finished attic. There are 3 bedrooms and a full bath up there. All 3 bedrooms have duct work and vents going to them but I'm the summer it gets way too hot up there. 2 of the rooms are bedrooms for the kids and then third I have made into an office for myself. I have window units in the kids bedrooms but haven't put one in the office yet. I'm really trying to find a solution that doesn't involve me putting 3 window units in every summer and taking them back out for winter. I have included some pictures to help demonstrate what I have going on.

There is a little door kind of centrally located that inside is unfinished and has some wiring, duct work, and access to an attic vent fan. This fan has not worked since I have moved in. I will replace it if you think it will help my situation, but my thought has been that it's behind a closed door and the space is tiny so I'm not sure if/how much it would even help. Cold air does come out of the vents when the AC is running, but it's very weak.

The house was listed as 2560 sq ft but they took the square footage of the downstairs and just doubled it so i don't know the exact square footage, but it is a little less than that.

Some of the current ideas I have had are

1. Get some of those in line duct fans to help pull the cold air up there.

2. Give up for now because maybe this AC unit isn't big enough for my house.

3. Replace my attic vent fan.

Any advice you can provide will be very much appreciated!

Pictures. http://imgur.com/a/4BmyI

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