I am typically a spur of the moment, plan nothing type of writer. I tend to just start writing, and develop the plot as it comes to me. Well, it hasn't been working for me too well until now! I've probably got 6 different beginnings to completely different stories that have run out of steam after I got the opening scenes off my chest.
I too have tended to be a spur of the moment writer and like you the stories ran out of steam. However, I persevered and finally have a plot line that I have continued to write until the end.
The only issue with writing a story without some predefined outline is that unless your careful you can end up with so many twists and turns you get lost.
A person told me once that if you want to write a book, try writing a one line description of each chapter, but start with the beginning and end, then fill in the rest. If you can't bridge the gap between the two try altering the plot or changing it completely.
I don't know whether this is helpful and I don't profess to be an experienced writer, but it's a method I have used which seems to work.
Above all enjoy yourself!!!