Not familar with volvo outdrives,but had a similar experience with my alfa drive years ago. The lack of lubrication and heat build up in the drive due to low rpm running at trolling speeds over the coarse of many hours may have talking its toll on your forward gear. This is why you still have reverse. On my boat I had to drive in reverse from the lake to marina,pull the boat,remove top cap only to find that the gear, bearings where basicly schorched from heat. If I remember correctly, about an eight hundred dollar repair. I do remember that feeling of trolling all day, pulling lines, powering up and then nothing. Note this does not happen in a day or two but over time. May or not be your problem but this is just what happened to me.