I'd like to be able to play on American servers more comfortably. What can I do to decrease my ping, I'm already using an internet cable. I'm open to buying a new modem if that should make any difference.
It is highly unlikely you can get any meaningful decrease by changing your equipment. In most cases it is less than 1ms delay caused by your router. But lets say you could do the impossible and reduce that to zero. 1ms is nothing compared to the delay in the network.
In perfect conditions you are still limited by the speed of light. You could calculate this if you really want to but remember ping is round trip time so it travels twice the total distance.
There is lots of overhead with all the equipment the signals pass through. The fiber paths never tend to be very direct either.
In general you will get about 100ms between the EU and sites on the east coast of the USA.
In the end you can do nothing about it really. If you were a bill gates you could have a private international fiber run between your house and the data center. Your only option would be to maybe get another ISP that has better paths but most people don't have that option and it may not be any better if the distance is the actual problem.