I have a friend who's been through a similar position, except in the hardware world. Last year he got a full time job, and while he says he still wants to pursue his idea, the likelihood is diminishing every day. Just for reference, he's a Ph.D. just above 30, and his idea comes from something he discovered coincidentally from his research.

If it is really a good idea, find a partner who complements your skills. Assembling a team is probably the hardest part. If you can't assemble one, all odds are against you, no matter how good the idea is. If you don't have people at your disposal, you have to convince other people the idea is good; you'll have to prove it some way or another.

If you're like my friend, who has extensive knowledge beyond what competitors would have in several years, the idea is unobvious enough that even if you outline it understandably, you don't have worries about competitors. If it isn't at that level of sophistication, my bet is that even if competitors haven't thought through things as thoroughly as you have, they will have implementations that are simply good enough.