Reducing voltage drop to shed
You are using the wrong wire

I ran about 90 feet of direct burial 14/2 wire from an existing breaker box from a 20 amp breaker to my new breaker box also 20 amp.

For 20 Amp, you normally need 12/2 wire, not 14/2. 14/2 is for 15 Amp circuits. Do you need bigger wire (e.g., 10/2 for 120 V or 10/3 for 240 V)? I'm not sure, that depends on how much of an issue there really is with the voltage drop, and also depending on what equipment you are actually using (or planning to use in the future) at the end of the cable.