Private pilot reported a loss of control when the aircraft's exterior door opened in-flight. The pilot was able to regain control of the aircraft; close the door and return to the origin airport.

Narrative

While flying in IMC at 2;000 feet following radar vectors with autopilot engaged in Heading and Altitude hold mode; I observed the cabin had become noisy and colder and saw that the door (there is only one door in my aircraft) was gaping open along the upper edge and that the latch was no longer in the fully engaged position. I attempted to pull on the door handle and fully engage the latch; which resulted in the door becoming fully unlatched and catching open in the slipstream. When this happened the autopilot was unable to maintain heading and altitude; which caused a deviation from assigned altitude and heading.Due to the disruption; pitch had descended through 10 degrees; almost to 20 degrees down with a roll to the right. I disabled the autopilot and [requested priority handling] due to the challenge maintaining assigned heading and altitude with the door unstable. I was also very aware of my proximity to the long line of commercial aircraft on approach to ZZZ - I had just passed 1;000 feet below a commercial flight as it made its base turn on approach. I wanted to be sure ATC was aware that I was having an issue.The area had some airports with very low ceilings and others with ceilings approaching MVFR. I initially diverted to ZZZ1; but after getting the plane stabilized was able to get the door partially secured and stable; so returned to my homebase of ZZZ2 for an uneventful landing.I was quite surprised by how much the door impacted the controllability of the airplane. While the door being partially open should not in itself be an emergency; the controllability issue created the potential for a loss of separation in the high density traffic area. I believe the issue may have been caused by a pressure difference created when I closed air vents that I have typically had open. The IMC conditions made the issue much more challenging to address without autopilot being effective.