Hello and welcome to the forum. We appreciate your question and are sorry you have this ongoing situation. Knee pain is very common although you are young to have it on a chronic basis. Something to look into is patella tendonitis. That you have pain going up stairs is a sign of this type of issue. Here is some information about that to review. https://www.mayoclinic.org/diseases-conditions/patellar-tendinitis/symptoms-causes/syc-20376113 This also continues to worsen over time. It can be an overuse injury. Little tears multiply and inflammation and pain increase. It helps to do gentle stretching as tight muscles are a risk factor. It's great that you walk regularly as staying physically active is important. I don't know if they've looked for this as you mention testing not producing any diagnosis thus far. Normally, this will eventually show up through imaging like MRI or x ray or ultrasound. Ibuprofen and other nsaids are often prescribed as short term pain management but other things like physical therapy can be very beneficial. They do also have second line treatment they can go to including everything from steroid injections to surgery. So, this is something to discuss with your doctor to see if it is a possibility.
Truly, stretching is one of the best things you can do overall and if you have knee pain due to tightness, this can greatly help. Talk to your doctor about this.
There is also something called patellofemoral pain syndrome which is an imbalance of muscles surrounding the knee which then affects the knee cap or patella. This is common in younger people. Yes, this is also blamed for 'runner's knee'. https://www.mayoclinic.org/diseases-conditions/patellofemoral-pain-syndrome/symptoms-causes/syc-20350792 But it also could be part of your situation as well. Please let us know if you can see your doctor again.
You're going to need to see someone more capable than a GP for this. You're going to need an MRI. You're going to need to see an orthopedic surgeon. There are several parts to the knee area, and it could be any one of them. Could be meniscus. Could be patella tendonitis. Could be cysts. You need pictures and a diagnosis based on what movements make it hurt. My guess is, you're going to end up in physical therapy.
Forgot to add my BMI is around 22 so it’s not because of weight