The meaning of the first sentence is that Lucy had not, since college, enjoyed herself as much as she was enjoying herself in the present moment. The word "since" in this instance indicates a passage of time between a present moment and a similar past moment and implies that in the passage of time between the two moments, no similar moments have recurred. In other words, Lucy had a wonderful time in college and is having a wonderful time now, but she has not had any similarly wonderful times in the passage of time between college and now. The phrase "Not since" means, in this context, "not until now," or perhaps more specifically, "not again until now." The sentence, therefore, might be rewritten as: "Lucy had a wonderful time in college, and not again until now had she had such a wonderful time again."
In the second sentence, the meaning is that the speaker did not fully believe that John was safe until he saw John for himself with his own eyes. The word "until" is conditional and indicates that something will or does not happen until, or before, a given condition is met. One might say, for example, that until a defendant is proven guilty, he or she must be presumed innocent. Or, to give another example, one might say that the game cannot begin until the referee blows his or her whistle. In the given sentence the something that will happen is the speaker believing that John is safe, and the condition for that belief to exist is the speaker seeing John with his own eyes. The phrase "Not until" means, in the context of the sentence, "not before but only when." The original sentence might, therefore, be rewritten thus: "Not before but only when I saw John with my own eyes did I really believe he was safe."