You've stripped the thread. It's a VERY easy thing to do, especially as they get a bit older. I stripped my engine oil drain plug last week, and I hardly tightened the damned thing at all> So I feel your pain. In my view a Timesert is the best answer. I don't know what sort of level of mechanical competence you have, but it is possible to do yourself. The kit comes with a drill bit, countersink, tap, and thread insert. You drill the hole slightly oversize, countersink and tap it, and screw in the insert. I always use two crush washers afterwards, as if you hand drill them they are rarely 100% square. A competent bike shop will do it. So long as you have good access to the site, its pretty painless. People may be along shortly to tell you its a disaster to do it in situ, and you need to get the casing out, but with some grease on your drill and tap you can avoid most swarf going in. And its only aluminium.