It just occurred to me that this needs to be said: if you start a song, just keep going until it's done. This is not the most critical piece of advice you'll ever hear, but to actually complete a song, even if you are not completely pleased with it, gives you some momentum to keep writing. I am in the middle of writing a song at the moment. It is not yet good enough, for me anyway, but I realize that this happens frequently. Actually, there are plenty of songs that I have written that I didn't think were good enough, but whatever I was trying to convey came across to others just the way I hoped they would.
How many times have you come up with a great hook, or a promising chorus, only to forget about it and move on? It's not so much that they aren't good enough! It's that they are abandoned, not knowing where to go from there. But I say to you writers out there: Just keep writing! Finish that crappy song! There is always room for revision and you may end up with something great, simply because you let your brilliant mind take over and just kept going.
If, when you've finished your song, you decide that it sounds like so much other music out there, don't beat yourself up. Give yourself a pat on the back for coming up with a sound that has a proven track record! Imitation is usually not the goal for songwriters, but sometimes you can't help where a song wants to go. Every successful songwriter writes plenty of regular-sounding songs inbetween great-sounding songs. Just let it go. Finish the song. Write another one. And another one. Repeat.