The Pole suffered major injuries in a rallying accident in February 2011.
Speaking to F1 Racing magazine, Kubica believes he can one day return to an F1 cockpit.
"I still believe I can come back," he said. "It's not just a dream. Right now I have limitations with driving single-seaters, but they are less than they look from the outside".
Kubica will take part in a DTM test in Valencia this week and believes such events have helped him.
"Things are improving. It wasn't like this six months ago and it wasn't like this three months ago. And thanks to rallies and tests on the race track, slowly my condition is improving - but there's still a long way to go."
"If you cannot have the thing you want, you want the thing you have. Most important for me is that I have seen the pace is still there and I am absolutely sure I can drive competitively again."
Kubica explained to the magazine that his "bigger problem is the limitation of the arm and the limited functionality in my fingers".
"Nerves need a lot of time and I see progress even if it's slow. I am quite sure this will be... not fixed, but not a big problem."
The former BMW Sauber and Renault star added that watching F1 races had not been easy.
"When you have races for 20 years and one day you have to stop... it's not easy, especially when you are hit with big problems".
"If I have luck and keep working and the puzzle comes together, maybe one day I will drive an F1 car," added the 28 year old.