Naturally this is subjective. I think I've written a few "good" songs that I think could compete with some of the most "successful writers" in similar genres. However, I've also written a couple of hundred not so good songs (some were downright pathetic). You have to write crap to learn how to write good stuff.
I've made a few bucks in the process with a couple of songs used in movies and a TV show. I've been paid for a couple of jingles and for composing music for a couple of indie films and a couple of stage plays and I've even writen music/voice over for a couple of industril training videos and music on hold.
While I've had some minor "successes" - I've had some miserable failures and I have a file box full of rejection letters. But, I take the craft serious and I keep trying, keep writing and hopefully keep improving.
Certainly the chances of major "success" are small and regardless of the amount of dedication and significant effort involved - there is a great deal of luck involved and I have no real expectations of anything more than small successes here and there (maybe enough income to by a piece of gear or two.
However, if we are serious about writing, we have to keep writing, learn from failures and accept successes with due appreciation. Am I confident in my abilities - I think any performing artist has to have a large dose of self confidence - but I hope I am humble enough to balance confidence with reality.
While much luck may be involved, I do know that the harder I work, the luckier I get.