why is it so much easier to start a new song than to continue one?? it's driving me crazy. you'd think continuing something would be easier than coming up with something from scratch, but no. i'm guessing you're going to answer with "try making shorter songs", but that doesn't solve the problem because it doesn't train the skill of coming up with "the next part" of a song. i think learning to play already existing songs on an instrument could help train that intuition, what do you think about that?
here's what happened to me. when i was at your stage, one solution is to just finish it in 1 session. i was writing entirely on paper at that time. but i soon got tired of that method (it's a good method, but it results in some pretty mentally insane songs, with all due respect to mental insanity), and i also came up with a lot of choruses, verse fragments, or just intros, and didn't know how or WHEN to deal with them. but here's what i eventually did: i went through all my notes and collected every song or song fragment i had ever written or worked on. and i put them all into a list in chronological order. Then, i just started at the beginning of the list, and 'find the next thing' or, just 'answer the next question' on each of them, before moving onto the next one. this unfortunately would take infinite years, so i shuffled things up a little and did a percentage completion system, etc etc, but still had ways of selecting which one i was doing next. nowadays i'm selecting to record, which means whichever one i select, could be a fragment, is the one i will record. so that puts me in a situation of writing for the recording which is about to happen. I guess what i'm trying to say is, the selection process is so important because if you are not confident about the validity of your selection, then you will run away from the song. So i guess the selection is one half of the challenge. then there's just techniques to do once you're there and you're already sure you're working on the right song, now what do you do? Well it's a lot like being a doctor. you just have to ask good questions. First thing i would check, is, 'is this the beginning', like, does the piece you have belong at the beginning of the song, or in the middle, is it the chorus, etc etc. If it clearly belongs absolutely at the total beginning, then, the process is pretty simple. Just run it through, and stop playing at the end of what you have, and wait for lightning to strike. Then if it doesn't, ask as many questions as you can about what you think you might have been about to hear.... Then what i do, for a re-set, is i walk over and touch the door handle, and come back and run it again from the top, and do the same thing again. It's extremely important that you run it AT TEMPO (at the right tempo) because often what we do in songwriting if we're trying to think of ideas, we run the bit before, over and over and we start doing it faster, which will trick you into then writing things at the wrong tempo and it won't fit with the rest. so make sure all your run throughs are at tempo. it's good to get on the metronome as soon as you can, to preserve, because songwriting is a time warp and it can make things change speed when you didn't mean to. But i didn't mention what to do if the piece you have is NOT the beginning. in this case you are going to have to do a weird backwards puzzle to figure out what comes before. It's almost impossible to figure out what comes before something, actually. one weird trick which might be semi-reliable is to run the chorus and see if you can get a second verse, and then hope that one of the things around there also makes sense as an intro, but at least for me writing backwards is almost impossible