If you are a discovery writer, then just write your story until you hit the end. If you have ideas for other stories, note them down, and then return to writing the story at hand. You will know when the beginning that you set out from wraps up and completes from your gut feeling of completion.
If you are an outliner, then figure out what the story is about. What is, to use an example, the mystery that needs to be solved? What is the goal that the hero tries to achieve? The end will be, when the hero achieves his goal or finally fails forever. The end will be, when the murderer is caught, the riddle solved, the world saved, the lovers married. I don't know what you are writing about, but if you are not just meandering through your imagination but are actually telling a story, then the end to that story is implicitly contained in it.
If I understand you correctly, you are not actually writing anything, just playing with your ideas. You need to decide on one and work on it. If you cannot do that, then you are not a writer but a dreamer. Writing, like all other occupations, needs a certain focus and discipline. If you cannot control your attention to stay focussed on one task, then you cannot succeed at that task. And no amount of advice for writers is going to help you, because then the problem is not in a lack of craft but in either your attitude or your ability to focus.