You've had two great bits of advice already, really worth following up!
I teach art in a college and find lots of students who start lots of work off but can't finish anything. What it is, is fear of failure, or more accurately fear of mediocrity (in their mind, though others will see more in their work than they do). You are avoiding finishing a song because when it's half done it could still become the greatest song ever. Once finished it certainly won't be. It'll be ok. Fine. But not great. But that's what you need to get over. This is one long process. The first song you finish might be aweful. The next one might be pretty poor. After that, poor, or even aweful again (to you). But after that, an average one. Then maybe a good one. After song 10 maybe you'll be onto something, a style, a feeling. At song 20 you might be writing a good song. Like the students making art, you need to see it as one long process, not see the pressure for each thing to be perfect, complete and the embodiment of everything you're going to achieve. You almost need to get those first 20 songs out the way without thinking or worrying about them at all, just like the others have suggested.
As has been suggested, set a time limit. Make a song. Simple pattern ; intro , verse, chorus, verse, chorus, bridge, chorus, end (or whatever). Make it. Play it a few times, put it to bed, start writing the next one and repeat. Just make em and dispose of em. No pressure. And then use a forum like this to road test one that you don't hate. You'll likely be the worst judge of your own work and that's why these forums can be so useful.
Good luck! Less thinking, more making!