my first British passport was issue on 1971 and expired on 1975. when grenada got their independence the british embassy told that I lost my CUKC's passport or nationality because my parents were born in grenada
And they are correct unless your father went to the British Embassy when he became Venezuelan and formally signed away his British nationality.
Did this happen or did it not? You need to find out the answer. Any Venezuelan forms your father signed at the time are irrelevant.
If your father did
sign away his British nationality at the British Embassy, then they are probably wrong in what they say, and you are a British Overseas citizen.
If your father did not
take that specific step, then you almost certainly became a citizen of Grenada on 7 February 1974 and lost your British nationality at that point. Whether you are still a Grenada citizen or not depends on what Grenada law says.
Paul has given you some very good advice on the next steps to take. It's your choice as to whether or not to follow them. But either way, no online forum is going to find out these answers for you.