you could try the folk tune finder, it might come up with what you think is your tune (It's just had a rebuild and it works a bit erratically for me at present). Otherwise - keep writing them, keep playing them. If one day the lightbulb comes on and something inside your head say "Oh s..t that's by Beethoven!" change a few notes and keep right on!
I write other peoples tunes sometimes. And sometimes I suddenly realise I've written this one before!
If you listen to any amount of music from a fairly short time span but from different composers - say written over 20 years - you'll hear a great deal that is almost identical.