Try and remove the door handle and replace is with a less cumbersome one. These can be purchased from local hardware shops are rather inexpensive. If you are prepared to spend more money you could change the door structure and have a sliding door instead.
If it doesn't also stick on the inside, I would try to find a can of WD-40 and use the little red stick that comes with it and attach it to the nozzle. Then aim and spray at the hinges under the handle.
Identify the specific part of the door that is sticking. Then file it down a few milimeters at a time, testing opening and closing, until it doesn't stick.