Dd is 4,5 and was bf until a year and a half ago. From the moment I weaned her (or soon after anyway) I felt that my boobs were mine, private, etc. To her, I'm guessing, they were just another part of my body, one that she previously had full access to. And she did seem obsessed. Or was the obsession mine?? This went on for a loong time.
Anyway, what worked for us, was that I de-mistified (?) the whole thing. I let her touch, and once even suckle. This did two things. 1, I wasn't uptight about my boobs any more. 2, she stopped constantly touching them (almost immidiately). Now she just pulls up my top and strokes my waist, lol.
So I guess my advice is, let it go, examine your feelings and deal with them. Talk to your child about times and places where it is NOT acceptable to touch others. In public etc.