Hi Mark
No, one of your own songs, that you put on SoundCloud, YouTube, etc. is not considered "published" and will not affect any future pitching opportunities
When you write a song you own 100% of the publishing rights to it - ie YOU decide what you want to do with it, where it is posted (SoundCloud etc.)
If the song were to start making money you would receive 100% of the publishing royalties - as in most cases 50% is paid to the songwriter (you) and 50% to the "publisher" - also you in this case
If you felt that an established music publisher (ie an expert in the field with lots of industry contacts etc.) would do a better job of commercially exploiting your music than you, then you can try to sign a "publishing deal", which usually assigns 50% of the royalties (often referred to as "the publishers share") to the publishing company, whilst you still retain 50% of the royalties ( often referred to as the "writers share" or "artists share")
All of this is only relevant if your song is generating money of course
If you do want to make money you need to pitch your song in the best way you can - and (IMHO and experience) the best way to do that is to get feedback on your songs and improve them using this feedback
Again, IMHO, the best way to do this is to share it using SoundCloud and use the reviews section in this very forum (and also review lots of songs, which will also help you improve your own writing)
Once you use this feedback and identify your strongest songs you can start pitching them to publishers
If you sign a deal your song is then what I would consider "published" - and your publisher will have a stake in your song and may or may not want it removed from SoundCloud etc. (mine is happy for them to stay there at the moment)
This is, of course, the first step on a long journey as you now need your song selected (often put "on hold" so nobody else can have it) "cut" (recorded by an artist) and then released - all of which can take a looooong time and fall through at any moment
Copyright is automatic (at least in the UK) so I wouldn't get hung up on protecting your copyright - the risk of someone "stealing" your work is extremely low and if you did write a "hit" a publisher would want you (the goose that lays the golden egg) rather than the song (the golden egg)
So, in short, get a song up on SoundCloud, post it up for a review in the reviews forum and dive in to reviewing other people (which will also increase the number of reviews you get
)
It can be a big step, but I am 100% sure your songwriting will improve, which will increase your chances of success