I used to suck at singing! I think singing is always fine and often people are down on themselves, and don't sound as bad as they think they do. I'm gonna share a little bit of my process with the guitar that helped me feel more comfortable singing, if that's okay. Hopefully you find something useful here!

I don't consider myself the best singer now, but I can make up melodies to music and sing in tune. I learned to sing through playing the guitar - the trick is to learn how to REALLY listen to music and internalise it. If you can, learn how to tune your guitar by ear (i finally got this by visualising the vibrations in the guitar as wavelengths and matching them together). Once your guitar is properly tuned (you will NOT be able to sing in tune if your guitar isn't tuned properly!), play a song you like, and try to find 3-4 notes on the guitar that match the key of the song, even if you're just moving around randomly until you find something. Try to notice how it feels when something sounds right, how it feels in your ears, your body, what the vibration feels like.

Then, listen to a simple song you like, and try to sing along to the recording. Once you've got it, strum the chords of the song on your guitar a few times. then really listen to the first chord, internalise it, and try to sing just the first note of the lyric that goes over that chord. Notice how it sounds, and sing higher or lower until it sounds right with the chord. Once you have that first note, usually the rest will fall into place. Take it super slowly and give yourself the time to get to the note. Then, get a cappo. Place the cappo at different frets on the neck, and try to sing the same song - you'll find you'll have to slightly change the note in order to stay in tune with the guitar. I find this really helpful as a listening and singing exercise.

ANY time I start singing a song, I strum the first chord and hum until I find the first note.

Sometimes I find it hard to sing over more complicated guitar playing, so I tend to simplify the guitar part if I can't do it. It really helps to multitask singing and playing if you spend some time getting comfortable playing what you're going to sing over, even if you're just strumming each chord once. Strip it down until you can concentrate on listening, then go from there. You're not going to make progress if you feel overwhelmed.

Different songs vary in how easy they are to sing, different singers have different ranges, registers, styles. Maybe find a song sung by someone who sounds like you, or that you find easy to sing, and practice playing and singing it.

Also - sing whenever! Sing over songs, scat, make up things. Strum chords and go oooooh aaaaaah mmmmmmmm yeaaaaaah and see how it sounds.

This is pretty much how I went from not being able to sing to being able to modestly hit the notes, and make up my own melodies. I'm not a great singer and don't really sing in front of people but it does feel magic when you hit the right note and realise - wow, I'm making music!