Now six weeks and pain is not improving!
I think that almost everyone still has pain at 6+ weeks after surgery. Even though you had a partial, not a total replacement, it was still very major surgery. You had skin cut, soft tissues pulled aside, bones sawed and large metal parts hammered and cemented into them. Then everything was put back together again and you were stitched up.
I wonder if someone along the way told you that you would recover fast from a PKR? Unfortunately, some surgeons do give that information, but it's not true. It usually takes just as long to recover from a PKR as from a TKR. Your age doesn't help either. Younger patients don't recover faster, unfortunately.
PKR or TKR, young or old, recovery from a knee replacement is a long process. It's a marathon, not a sprint.
Did you contact your regular doctor, to see if he/she can prescribe a pain medication that is not a strong as the Oxy? I think someone suggested Tramadol as a step-down pain medication. If you can't get that, taking a regular dose of Tylenol, which is an over-the-counter med, can help a lot. The effective dose of Tylenol (Acetaminophen) is 1,000 mg (1G) 4 to 6 hourly, to a daily total of 4,000 mg (4G) in 24 hours. Split that daily dosage into 4 doses.
If any of your other meds contain Tylenol, factor their amount into your calculations of daily maximum dosage.