Colwill won the U21 Euros a year ago before making an impressive 32 appearances for the Blues.
"I think it's been a crazy year," said Colwill to chelseafc.com. "Winning the European Under-21 Championship last summer was amazing, it was one of the best things I've ever done in my life. Just being there with the boys was so good.
"To go from that to then coming back, making my Chelsea debut, signing a contract for Chelsea, all of the things that I've dreamed of from when I was a little kid across the road (at Cobham).
"Then to make my England senior debut, I still don't really believe it. I still don't feel like it actually came true.
"I've had ups and downs. I struggled as I played in a different position for half the season (at left-back). That was really tough, but I learned so much from it."