Cape Cod style house?

I woukd start by figuring out what sort of insulation you hazve in your ceiling and it's r-value. You can find large insulating panels at places like Lowes or Home Despot.

In the summer, close off your downstairs supply vents most of the way. Make sure the upstairs vents are open all the way. This will help push the cold air upstairs since cold air drops and warm air rises. In the winter close off your upstairs vents most of the way and be sure the downstairs vents are open fully.

Upstairs hot in the summer / cold in the winter.

Typically problems of this nature are design related. Things were done (when the house was built) to cut costs. The design of the HVAC system was one of them.

You don't say how many HVAC systems you have. You don't say how they are controlled. Insulation is important, but without proper control and design of the HVAC system an insulated house is still either too hot, or too cold with a poorly designed HVAC system.

You have to look at the whole picture, not just bits and pieces of it.

A two story structure is best heated and cooled by two separate HVAC systems. (each floor is zoned, via equipment selected. Returns pull air from each zone. The equipment is controlled via thermostats on each floor or area of the structure.)

In a cheaper build, a two story structure is heated and cooled via one HVAC system. No matter where you place the control to operate the HVAC, one area will suffer (either too cold or too warm) Lower or raise the setting all you want. (this is what is meant by design limitations.)

Maybe you don't have the room for a second HVAC system? that falls into the design of the home. Cheaper build kind of problems, cost of land, cost of materials etc.

HVAC zone system might prove effective, but these systems are known for not being implemented properly. These require skill as well as "design" knowledge. They work when implemented properly. I own two homes with zone systems, have solved several others with zoning. But realize zoning is a "tool" in the tool bag. Not all homes are good candidates for such systems. (anyone can sell you something, I don't operate that way - I am known for solutions that work.)

Provide more information of what you have, where you are located as what I have listed above are merely guesses based on 27 years of doing this for a living. 14 of which I have owned / operated my own HVAC company.

I service the Katy, Texas area.

Insulating slope ceilings has its challenges-- even more so after the finish material has been installed.

You could add rigid foam insulation sheets to the underside of your existing drywall ceiling and inside of the walls and then apply a finish material over it. It will, of course, lower the ceiling height and reduce the room width according to whatever thickness you use.

The other option is to add insulation above the roof decking, install sheathing and re-roof your home. That's an expensive alternative, but one which addresses the root cause.

With or without additional insulation, you should consider either zoning your existing system or installing a new system dedicated to the 2nd floor. In either case, get a competent HVAC design professional to run heating/cooling load calculations for each level and size the required equipment and ductwork.

As stated by @Austin Air Companie - you can only "push air" so far...one floor above the furnace is the limit. Consider a mini-split unit (Mitsubishi / GE / LG - dual AC & heat) for the attic.

A properly designed HVAC system can serve multiple levels and maintain occupant comfort, too. I lived in a two-story home in PA conditioned by a single air handler located in the basement. The rooms on the second floor were comfortable year 'round except for the room over the unconditioned garage. That's a space that heats up in summer and cools down in winter faster than the living areas over conditioned living areas. It's a candidate for a zoned system. We find conditioning such spaces with mini-splits is usually the most economical option.