On the whole, it sounds like your AC unit isn't up to the task of blowing enough cubic feet of air to cool your entire house.
You could do one of two things.
1.) Get a new AC compressor unit.
2.) Close the downstairs air vents. All or most of them. This will force the cold air to come out of the upstairs vents. Eventually, the cooler air will drift down to the rest of the house, time/coolness depending on how your house is laid out.
There's no point in cooling downstairs if you spend your time upstairs. Much less sense than if were the other way around b/c hot air goes up, cold air goes down.
The only downside of this is that your overworked compressor gets even more of a workout and you lose overall cooling efficiency because that air has to go from downstairs, where I assume your compressor is, and push that mass of air all the way up.