The man that got so much down already, Mazz, had a great answer, I think. It's important to lay down a few ground rules for yourself, now that you know you are a creative person with these challenges. Actually I don't think it's a character trait at all, like the MBTI, Belbin ect tests wants us to believe. I think we are challenged with this, because our craft is not fully down (will it ever be..). Craft is supposed to help us in these situations. And what is craft? It's systematics, templates, forms ect., so I suggest you let go of the personal blame, and just focus on setting up some crafty rules that'll work for you (instead of against you):
First ground rule - clarify the song/track idea, so that you actually know when the track is 'finished'. Often we just fumble around with things, looking for ideas, and then when we've found an idea, we wonder why it's not done. So it might be counter intuitive, but working more on formalizing the idea, like Mazz points out, might be a great way to go. By doing that, you will not endlessly looking for new ideas, but actually have a vision for a finished track.
Second ground rule - clarify the arrangement, so that when you improvise, you do it within set parameters. Same goes for choice of instrumentation/libraries. There's just so many tools at hand, that it'll take you a full month just exploring the basic stuff of a new gadget/ library. Use reference tracks to guide you, and just make a decision on what will drive the rhythm, what you will use as a pad/textures, what you will use for melody & counter melody, and what you will use for enhancements/fx. Then you'll have a vision for yourself, that'll help you know when the music is finished.
If you then are not happy with the results, make it as a rule to spend more time envisioning the music, rather than continue the experimentation. In stead set off separate time for experiments (learning new software, instruments ect.), where you don't require a product from yourself. Same goes for research. Mixing those processes can be fatal.
That way you also know if the music didn't end up like you envisioned, it's not you, it's the vision you put up for it. You can start working with concepts and of course, the Taxi listings are great for this stuff.
Finally, you'll probably never end up with exactly what you envisioned. That's not a mistake or a flaw, THAT's what makes your music unique! I think accepting that is the biggest secret

justmy2c