I empathize with your yearning to sing while feeling unable to do so to your own satisfaction, even though I grew up singing, around people who sang, in a church where the congregation sang. Your (relative) youth gives you the advantage of time to develop this skill, and it IS a skill, like any other that, need practice, endless repetition, attention to how your body is working, and an encouraging atmosphere.

A part of my early "lack of ability" was a combination of untrained, rudimentary skills running up against being overly critical of myself. At my age, we all expected we should be able to sound like Joan Baez or Judy Collins (as if). I was well into my 30s before I tried to sing solo as you are, with no backup. I loved LONG songs with many verses, and had plenty of scorn thrown at me for "how many different keys did you sing that in?" My solution was to get a simple instrument to accompany me: in my case an autoharp. It kept me on pitch & if I needed a specific note to try to match, I could. While a whistle sounds fun for making music, problem is, you can't SING WITH it. The musical prop worked in both improving my ability to sing on pitch, in tune AND I learned how to play it as an instrument (over 25 years, and I had a late start).

As has been previously mentioned WORDS - find songs you SPECIFICALLY want to learn. Write out the words. Sing them. Sing them again. Sing them LOTS. Listen to yourself, sing some more, figure out what you can do better. If you an find a recorded acapella singer with the range and repertoire that suits you to sing along with, use that.

Find an ENCOURAGING group, person, place who can help you with critical input that won't crush you. I almost gave up when a noted singer, himself NOTORIOUS for messing about and restarting a song several times before he could find the right pitch, told me in a ballad class, after one song, that "we didn't need to hear any more" HAVE FAITH. Ten years later, same guy, same workshop place, open sing. I started a trad song I loved, he was among the folks who sang along and said "Oh, that's grand! I haven't thought of that song for years!"

Keep singing. Keep trying. Find out what works for you. See if you have a musical friend who can help you take this journey from "out to sea" to at home with your voice. The pleasure in singing is unsurpassed. I still feel frustration around the youngsters who can sing like a dream, parse out harmony without a thought, have repertoires that make my head spin. They're not the norm. They're not what one compares themselves to. Best of good fortune to you finding your voice and singing with it.

Joanne in Cleveland