i've been writing riffs and almost whole songs on and off the past few days but i'm really struggling to finish them and i'm struggling to turn my riffs into full songs. i feel kinda lost :/.
If you write for only yourself, it's A SHIT TON harder. My advice, play, jam, write with other musicians. Even if it's not a group or band or project or whatever, just get others opinions and viewpoints to help finish the ideas.
I heard Bumblefoot say once "songwriting is like taking a shit. Sometimes they slide out without a problem, sometimes it's a struggle."
Sometimes you just need to force yourself. Try taking your riff and coming up with a counter riff to it that is different but complimentary. Learn some classic Sabbath tunes for inspiration, Iommi is a master of this (even a tune as simple as Paranoid or Iron Man have great examples of riff A/counter riff B sections). Same with Death, Celtic Frost, and Opeth, just to name some of my favorites in terms of great riff structure.
A song's never really finished for me, it's at what point you decide that enough is enough and just leave it that is important. If you've said what you wanted to say with what you have got already, then you're already done.
I find it easier to write when you have a goal for a song in mind. For example, if you're going for a Lamb of God style song, you're typically going to stay away from longer and more complex instrumental sections and write more tense riffs with contrasting tense riffs (if that makes sense). Knowing what you want to do with a song and having any kind of goal in mind will help. Also, breaking the song up into sections based on "has the effect been established?" rather than by riff can make it easier as well. For example, if you have pieces (not chords) A B C that make up the intro to a song and you have A D for a verse and E that make up the chorus, you'd have something like this:
A B C - A D - E - A D - E
You may find that you like it but that something is missing on the second verse, so you add a third riff "F" so that you'd have
A B C - A D - E - A D F - E
Writing everything out on paper or recording it will give you an idea of where you stand. It's much easier to hear how a song should play out when you aren't playing it.
I find it easier to write shorter songs and add to them than to write full on songs in a single go. My first complete song started out as 3 piece song that looked like this:
A B C A B C A
After we recorded it, we didn't like how fast it got to the chorus (C in this instance), so we added another "A B" before the chorus. Then, we found that we didn't like the transition from C to A, so we added a 2 bar "D piece" after the C. Next, we felt as though the song was too straightforward, so we added pieces E F G and H, which made up a transitional 2 bar piece, a breakdown (sorry), a transition to a solo section, and a solo. After a few more months of analysis and re-labeling the pieces, it looked something like this:
A - B C B C - D - E - B C F G C - D - H I - J K L - M - N P - D - A
All of that came within about 6 months, with refinements taking about another 6.
It all really comes down to getting a base for a song. The next time you write a piece, you'll know exactly if or where that piece will fit in with your existing song. I always have 2 songs that I'm writing. Anything that I feel can stand on its own is recorded and saved until I finish with the 2 songs at hand.
Lastly, always carry something to write with or get (the Super Note app) because you never know where inspiration will strike.