I'm going through the same thing. My condition cleared up when my dermatologist prescribed me hydroquine 4% and chemical peels. However, the darkness returned once I stopped getting chemical peels and ran out of hydroquine.
Yes the problem is you will have to continue any treatment for life. Now I maintain my neck color with a new routine I have created. I wash my skin, then I use a product called maxi-peel exfoliant 3 ( which contain tretinoin and hydroquinone 4g),its a liquid that I put on a cotton ball and scrub the skin you can see all the excess melanin coming out, the cotton ball will become so black! I buy maxi-peel on ebay. Then I put the retin-a cream in small quantity and try to spread it as much as I can. Now as a skin lightener I use Fair & Flawless 5xf Body Lightening Gel to finish. This gel is expensive but its worth it. The 2 oz will last you for a long time if you use it for your neck only. This bleaching routine is the strongest I have ever use in my life, and trust me I've tried them all. My neck became so light that it was now lighter than any other part of my body even my face. I had to use a little of the fair&flawless gel on my face to make them look the same and now I am too light. Not a day goes by without somebody asking me (not even asking, the assume: you're mixed right!) They say I am half white and when I say i'm not they're suprised. I became almost white so I had to reduce my dosage of everything and only use my 3 products routine 2 or 3 times a week(out of 7 days) but thats because I maintain at first it was more frequent. So this is the first time I shared my secret. The only reason I did is because I know how hard it is to have a darker neck and I was always wearing turlteneck sweater all the time. NOT ANYMORE! At one point I even tried monobenzone (yes I was that desperate) but forget it it BURNS SO MUCH! OMG! It was hell for me pure torture! Now I have great results without the pain!