Not all dreams are possible or realistic. When this happens you have to bade them farewell. This isn't failure, but it's a sign of maturity and learning on your part. All you need to do is rethink what you wanted to do, in the first place,and see what you can change.Then that is called determination and that, will lead to you having success.
Have you made a choice in your past which was right at the time, but once you had a chance to think about it, you wished you could reverse your decision or amend it? Rethinking gives you permission to use your thoughts to change your mind. Take what is and spin it around to give you a new review and fresh perspective.