Emily is definitely right. In most cases those who have PKU that are not treated (especially in infancy and early childhood) usually have developmental issues. In your case because your phe levels were borderline you probably have Hyperphenylalaninemia (hyperphe), which is a mild form of PKU. This would explain why you have not had any serious issues.
Like Emily said your first step is going to be finding a clinic. Speak with your niece's geneticist first. If you live relatively close to your niece there is a good chance that you can be seen at the same clinic as her. If for some reason you cannot attend your niece's clinic you can either use the Clinic Finder link here, or let us know where you are from. We would be happy to help.
Once you find a clinic to attend make sure they are aware of your symptoms, and then you will need to get a blood test done to check the level of phenylalanine in your blood. From there the geneticist that you see should be able to help you with getting the diet started.
If possible it would be a good idea to try and go on the low protein diet and drink the metabolic formula. Will it be easy? Probably not, but it will be worth it. In many cases those who have been off diet and go back on say they feel so much better when they are on the diet. A lot of people who are not on the low admit that it wasn't until they went back on the diet and started feeling better that they realized all the symptoms they really had. Being on diet will also help eliminate any other future problems that might occur.