I think Dave is right, it sounds like you've got the wrong headtorch. Get down to a decent outdoor shop and give them a try. There are a number of different features that will affect the fit - separate battery pack, top strap etc. A bit of trial and error in the shop should sort you out!
When I moved from FR10 to FR310XT I removed the FR10 from Garmin Express. Open garmin express and click 'Device' (top right of window next to 'Updates'). This then gives you an option to 'Delete Device from Express'.
That worked for me. Having said that the FR10 transfers data over a cable to there is no danger of it being picked up accidentally by Garmin connect.
I had a Forerunner 10 for just over a year and I thought it was brilliant. Simple to use, reliable and accurate. I've just got a Forerunner 310XT and it is even better - even though it's got many more features I'm actually finding it simpler to use! I got it off amazon (with the heart rate monitor) for £124. The Forerunner 10 is on for about £80. If you can afford it, the extra 40 off quid is definitely worth it.