How do I make this injury better?

It depends what the pain feels like as to what sort of injury it is. It sounds to me to be muscular related. You should get some ibuprofen gel, and some deep heat spray or a cheap alternative. The gel will be a localised anti-inflammitary which shall take down the muscle ache by de-swelling the muscle strain. The deep heat will sting like mad when you first apply it, but it works for hours and its like having a soothing heat pad on the area, and will relax the muscle and stop the pain for you. To allow the injury to heal itself, which it will do, ou need to rest, so its best that you ease off the excercise and training for a couple of weeks. If you dont, it will still heal eventually, but keeping strain on the muscle will obviously slow down the process. I wouldnt worry about it too much for now though. Just try those pain remedies, and if after another month it hasnt gone away, visit your doctor. Also see your doctor if it gets worse. Hope that helps and you get better soon x