Some folks just have an ear for what to sing over a chord progression. But if your ear is failing you - I offer a formula below that you can use - even if you are unable to pick the notes intuitively.
Creating a vocal line is pretty much the same as creating a melody line and can be done with the aid of any one of the instruments you play. You might want to start by using an instrument to play the melody notes as you derive them from the methodology outlined below - and write them down and then you will be able to play the melody and sing the melody notes that you played.
Here is a simple formula that will enable you to write a melody that works for any chord progression you create or use.
Start by limiting the notes you use to the notes that are in the key of your song. That is not mandatory, but will always keep you in safe territory. This would limit your note choices to 7 notes but any of those could span one or more octaves.
Next (as you have mentioned that you have done) create your chord progression in the key you are composing in. Then for the melody notes that are sung or played over a particular chord, start with one of the 3 or four chord tones (notes used to form the chord) for that chord. The first note sung on a chord change does not have to be (but can be) the root note of the chord, but you will find that most melody lines will have a chord tone as the first note sung over a new chord at (or right after) the chord change. This is not an absolute rule, but will serve you well as a guideline.
Any of the chord tones will go with the chord but to give your melody more interest, you can add a few other notes to sing over the chord. To be safe, try to make most of the notes (maybe at least every other note) sung over a particular chord one of the chord tones.
One way to narrow your choices for notes to sing over a chord that are not one of the chord tones - is to choose notes from the key that corresponds to the chord but that also fall within the key of your song.
So for example if we are composing in the key of C major our major chords are C, F and G. When playing over a C chord we would choose one of the chord tones as the first note sung over that measure of C (C, E, or G) and the non chord tone notes we sprinkle in could be any note in the key of C which is any of the 7 notes we already narrowed it down to in step one.
Let's say the next chord in our progression is F. The same principle will apply for using one of the 3 notes in the F major triad (chord) as our first note in this measure. That gives us a choice of F, A and C. The same guideline applies to make most of the notes in that measure either an F, A or C (any octave).
The other notes available will be derived from a combination of the F major scale and the C major scale. We want to choose from notes that are common to both scales. We want to keep our notes within the C major scale because our song is in the key of C and if we are singing or playing melody notes over an F chord, they will sound better if they are in the key of F.
The notes in C major are C,D,E,F,G,A,B. The notes in F major are F,G,A,Bb,C,D,E. So the notes we should not use while playing the F major chord is Bb because it is not in the key of C and B because it is not in the key of F major. This leaves the notes common to both the C major scale (key of our song) and the F major scale (the chord we are now playing) which would be F,G,A,C,D and E.
The same logic will apply to the other chord sequences. Again, first note played after the chord change should be one of the notes that form the chord. If it's a four note chord (7th, augmented, etc.) you can use any of the four notes and often the note that makes it something other than a triad will be suggested strongly as a melody note or two while playing that chord.
If you must use a formula, a safe bet is to choose the notes that are common to the chord being played AND the key of the song. In other words choose from the list of notes that are common to both of these two scales. There are exceptions that will work in some circumstances, but this will keep you in safe territory.
For the minor chords, choose notes from the minor scale corresponding to the chord and use the process describe above to boil it down to the notes common to both the key of the song and the key suggested by the chord.
To summarize (TLDR)- the chord being played in a particular measure will give you a basic framework for notes to sing with that chord. That framework is supported primarily by the chord tones of the relevant chord and can be decorated by a few other notes common to the key corresponding to the chord and the key of your song.
Hope this helps.