How can I regulate brightness of a LED strip with a potentiometer
A circuit using a potentiometer and transistor attempts to regulate brightness by adding resistance in series with the LED strip. As you've noticed, it interferes with the maximum brightness. Also, as you dim the strip, you'll be wasting energy as heat in the transistor.
It's much more efficient to regulate brightness by turning the LED strip ON and OFF at a rapid, repetitive rate. To control brightness, you vary the percentage of time that the strip is ON versus the time it's OFF.
You can buy one ready-made (like this). But you'll learn more if you design your own. Here's an article with a simple design using a 555 timer and an NPN transistor. Have fun!