Hi Andy, this is my life's number ONE challenge as an extraverted HAP (highly attuned) and HSS. I have a love/hate relationship with my ability to be creative and distracted at the same time.
To add to what others have shared on this post: I scribble my ideas down onto paper the size of that found inside a fortune cookie. For me, the more writing space available, the further away I get from my initial idea. To rectify this problem, I journal; hence, my fetish for stationery :-) It may sound constricting but it has enabled me to stay focused.
Also, I share my ideas when I get the opportunity. I seek to network with people who are equipped with the financial resources, entrepreneurial spirit, and leadership. Then I literally throw my idea(s) and with my fingers crossed, hope they'd run with it, and bring it to full fruition. (So far two of my bigger ideas materialized; both are now social enterprises :-)
You mentioned that you have an idea (or more) that you keep to yourself because it is only useful to you. Hmmm...I've been ruffling through my box and slightly perturbed that I haven't found one that appeases me--so far all my ideas are for the betterment of others (and I mean this in an empathetic and compassionate way.) This explains why I'm in the helping profession-to enrich other people's lives; neither my mind nor my hands won't stop thinking or fixing, respectively.
Of course, the downside to this is that I neglect myself. The only time I stop thinking creatively is when I'm mentally and emotionally drained. If it weren't for my support network, I wouldn't be able to continue with my creatively thinking streak. Yes, it's challenging; but it brings me so much joy and passion!