My holy brother,
I assume that when your Rebbe suggests that you not share negative things about yourself, with the person who you are considering marrying, he is referring to minor character flaws, some kinds of bad habits and the antics of your youth. In general I assume he is referring to things which are insignificant, will have very litter impact on your relationship or your mutual happiness. However if the woman in question heard about them she might get anxious or turned off.
However I am certain that he is not suggesting you do not share serious medical issues or other major issues which you are halachiakly obligated to share.
Regarding the matter at hand I think it falls somewhere in between. You can shop around for a Rabbi who (does not understand the progressive nature of this addiction) and will tell you that you do not have to mention it. However Rabbi Twerski is of the opinion that you must share it with you potential spouse.
www.guardureyes.com/GUE/RTwerski/Dating.asp I want to add that many people who are dating have shared this and the other person, both male and female (see Rabbi Twerski's peice) understood.
I want to make a few points why you should consider sharing it.
1. Marriage and all other relationships are built on trust. Ever if we are totally powerless over this, if your wife catches you she will feel you have violated her trust. Just yesterday someone posted here that his wife found out about his struggle, she threw her ring at him and said she wants out.
2. This is progressive. You will get worse. Many people who never thought they would have gotten involved with other woman in very perverse ways.
3. Even if you hide it forever it will have a very negative impact on your marriage. You only need ask any guy who is married here. Marriage makes the struggle worse not easier. You and your wife will be miscible and then you will self medicate more and create a cycle that will only make both of you farther and farther from each other.
This issue has been disused at light before and I suggest you search the archives.
I am not trying to be harsh on you, only trying to help you be happy in marriage.