Could be number of factors causing it to get worse as it's probably best to see your GP again about it as they can prescribed better medication etc for you to try.
Another option is try your local pharmacy and speak to the pharmacist as they can recommend things as well if you don't want to go to GP. Boots generally sells a good range of products for this, as maybe cheaper over the counter than paying for it on prescription if you live in England.
Why is my seborrheic dermatitis getting worse.....
It is often made worse by cold weather, hormonal changes, and stress. Symptoms can include skin that is bumpy, scaly, greasy, and itchy. Treatment such as medicine in shampoo, body wash, and lotion can reduce symptoms
How to treat dandruff yourself
Use an anti-dandruff shampoo. There are several different types you can buy from pharmacies or supermarkets.
Look for shampoo containing one of these ingredients:
selenium sulphide (or selenium sulfide)
Your pharmacist can tell you how to use the shampoo.
Use the shampoo for a month to see if your dandruff improves. You might need to try more than one type to find one that works for you.