I think that you're using it on charge most of the time is part of the problem. To keep Li-ion batteries in top condition they should be discharged and charged up occasionally. It's probably "forgotten" that it's a battery!
For best results, the discharge/recharge cycle should be performed occasionally FROM NEW. You may be able to improve the reduced capacity with a few more discharge/recharge cycles, but perhaps the damage can't be reversed. Have a try and good luck!