There's a few things that I cant seem to shake when it comes to songwriting, and I'm wondering how many people are in this same place.

Im good at creating interesting music, bass lines, chords, generally all the elements I need for good songs and I'm more or less fairly proud of the music I can make... BUT, I cannot seem to step out of that zone and come up with a vocal melody or an interesting rhythm every time. Which means I have stacks of unused song ideas.

This means that for the last... year, I suppose, a lot of the music I've been trying to write hasnt had a chance to become a finished thing, and so I dont think I've really gotten anywhere. I know a few singers and it's always fun to write something with them, but their musical style is so far detached from mine that it just isnt stimulating enough.

I dont know how to move forward really, any ideas?
Start writing lyrics. Just flex your lyrical muscles. Yeah, your first few attempts will suck. But keep doing it. The only way to get better is to do 3 things:

1. Practice.
2. Practice.
3. Practice.

It's like when you first began to play guitar. You sucked. (We all did. Ain't nothing to be ashamed of, man.) So, you kept working at it. After awhile, you became better. Same thing here.
I'm the same way, but it's getting better as I slowly write the most ridiculous lyrics I can think of, I start to finish songs then get better at doing lyrics as they come.

Also my favorite advice was its not supposed to be finish it and its done, you can always change lyrics later.

As for getting a melody for a song, I had a problem cause I sang bass in choir, so I could never make any melodies for my voice. What I did was record my verse on my ipod (or any recording device) and then played something over it with my guitar that sounded good, and then I repeated it and and saved that. That was then transferred over for my voice melody. works great for me cause I can do that easily being able to make music much better than fill in with my voice.
Let me ask you a question:

Are you finding these basslines, chord progressions, etc by jamming around on the guitar until you find something that sounds good, or are you hearing something in your head and finding that on the guitar?

If it's the first thing, that's your problem. You need to train your ear so you can think in music, and then you write music by THINKING, not by experimenting. You may experiment once you have an idea that started in your head, or you may find a small idea by experimenting and then expand it by thinking about it, but your brain is the most important tool you need to create music.

You can't do it, however, if you don't have well-trained ear.

That being said, however, part of the problem is that you're trying to come up with melody last.

Melody has to come FIRST.

If I sing a melody for you, how many songs do you think you can recognize? I'm willing to bet it's in the hundreds if not thousands. With a bassline, it's lower, but because the bassline carries a melody in some songs (eg, "Walking on the Moon" by the Police) you can probably recognize a reasonable number. Same with guitar riffs - if the riff carries a lot of melodic content (eg the opening of "Sweet Child of Mine") you probably can, if not, you probably can't. How many songs can you recognize if I just play you the chord progression - especially if it's devoid of rhthmic content?

I'd wager very very few, compared to those other numbers.

The melody is the song and the song is the melody. Eg, I sometimes play and improvise off the melody of "A Day in the Life" and nobody who listened to it would ever say I played "part of A Day in the Life." No, they heard the song.

In my experience it is MUCH easier to write chords from a melody than to write melody from the chords. I've got some chord progressions I've written which I really like - but I have a very hard time fitting a melody to them. On the other hand, if I come up with a melody, finding chords for it is relatively easy.

Train your ear if you want to be able to write melody. Then once you have a well-trained ear, study songs that you like - sing them, learn how to play the melody lines on your guitar.
By analogy, when a player tries to first learn how to solo, one of the best techniques is to try to dissect a favorite solo note for note until you have figured out how to play the licks.

Similarly, it is incredibly difficult to know how to write vocal melodies (and full songs) without knowing how a song is constructed from the ground up. I think a good practice technique is to pick a favorite song and first write down the song structure. Ex) verse, bridge, chorus, refrain, etc. Once you have the structure of that song down, next listen to how the vocal melodies work over the various chords. Listen for rhythm and how the melodies are varied. Most incredible songs avoid repetition by varying the melodies and creating moments of tension and release.

After understanding all of the above, I would suggest taking one of your favorite song demos and trying to fit it within the structure of your favorite sample song. As the previous response said, it is vital that you finish your song.

Regarding vocal melodies, don't be afraid to try an enormous amount of melodies (along with different lyrics) being sure to focus on rhythm.

Good luck !