No, don't tell your would be spouse or to anyone for the Prophet has said,
Every one of my followers will be forgiven except those who expose
(openly) their wrongdoings. An example of this is that of a man who
commits a sin at night which Allah has covered for him, and in the
morning, he would say (to people): "I committed such and such sin last
night,' while Allah had kept it a secret. During the night Allah has
covered it up but in the morning he tears up the cover provided by
Allah Himself. Bukhari and Muslim
Once you have asked God sincerely for forgiveness, it's as good as not having done the deed in the first place. You shouldn't let the guilt bother you - and why should you feel guilty when God has forgiven you?
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah
forgives all sins. Indeed, it is He who is the Forgiving, the
This is also one of the reasons why the Prophet instructed us to separate the beds of children when they reach 10 years old. As a future parent, it's your duty to ensure your children don't go through the same experience as you did. Finally, if you discover your cousin is still in this acts, then you should ask him to desist. I know this is a very difficult thing to do, maybe you could get the message across to him through some other way, without doing it in person. God will make it easy, in sha Allah.