"anyone here has similar experience?
does intimacy even sex before marriage ruin it all?"
An elder once told me, not realizing the full implications of what he was saying, that "for all intents and purposes, when you become engaged, you are married. You are commited to that person."
And as such, my wife and I found ourselves in your situation, but with out the guilt. We made love on many occasions. And when we were asked about our conduct during courtship, with clean consciences we were able to reply that we were chaste. Our personal life is between us and Jehovah.
What is currently ruining it for me is not guilt but that I feel the way you do in having doubts about the WTBTS. My wife doesn't know this yet, and I'm trying to find the words to tell her. It's treacherous waters dealing with these feelings after the fact because you can lose so much in such a short time. Personally, I've been reading Raymond Franz's two books, Crisis of Conscience and In Search of Christian Freedom, to get a solid foundation of what to believe. Late at nights I have the books and the WT Library open side-by-side verifying everything he says as fact. As a former Governing Body member that started where we are, he knows what we are going through. I just wish I had the opportunity to meet him before he passed away.
So from PERSONAL EXPERIENCE, do not feel guilty about your actions during courtship. If you do feel guilty, not because of what the WTBTS says but because you feel what you did was morally wrong in Jehovah's eyes, then repent before Jehovah in prayer and make a commitment not to do the same. Do something that might not be wrong in itself, but leaves you with a bad conscience, is still a sin. I forget the scripture where Paul tells us that.
Also from PERSONAL EXPERIENCE, the real issue is whether or not you share the same views on the WTBTS. The Society is right when they tell you to marry only in the Lord because since it is difficult being yoked to an unbeliever. That's goes for both sides of the fence. To be truly happy, you need to share the same religious veiws when one of you is determined to be a JW, unless you are willing to face the repercussions. Trust me, it is killing me. Only 350 pages of In Search of Christian Freedom until I have my thoughts together.