Although this post mainly deals with lyricists, I think musicians can glean some truth.
*NOTE*... Most of what I say here is subjective....This is the way I see it.
I remember as a young boy, finding a well worn note binder on my mother's dresser, and being curious, I opened it and looked inside. It was filled from front to back with poems she had written; with the time written, day, and year at the bottom of each one. I sat down on her bed and started reading them. I had read about four or so, when my mom came in and caught me; mad as a hen that I invaded her privacy. After getting a lecture about how those were her private thoughts, I asked her why she never read me any poems (she read me books all the time). She said she didn't write them for other people, she wrote them to make her feel better. At the time, I didn't grasp what that really meant. Now I do.
Why do we write? What is your core reason to write? There could be many answers to that, from many different people. But the core reason should always be, because you enjoy it. Other reasons might be; "because I want to get famous" or "I want to make money at it" but the core reason is enjoyment. If you don't enjoy it, you're not going to get famous and it's doubtful you will make any money. Yet there's another reason linked to the enjoyment factor (at least for me) and that is; it's therapy. It makes me feel better. I can vent anything through writing. I can let it all out, and in the process, deal with what I'm feeling in a constructive way. So I encourage new writers, don't write in hopes of impressing someone, or for getting a lot of positive reviews. Because the more you write for that reason, the less positive reviews you will probably get. If you really love writing, you WILL get better. Yet, part of getting better, is getting in touch with yourself. Write what pleases YOU and advance in your craft from there. Listen and learn from the seasoned writers, and use their input to express who you are better, in a better way than you did before. When you're feeling depressed and can hardly move; write. When you are feeling frisky; write. When you're content; write. You get the picture. Throw off your apprehension and lack of confidence, and write to feel better, not worrying about other people. And then when you decide to post, and you find you get negative reviews; don't sweat it. The core of you is in the lyric, now just reword it into a polished form that fits a lyric criteria. The more you write and take advice, the easier it gets, not only in writing a good lyric, but also in being able to reach down inside yourself to pull one out. To sum it all up; write what feels good to you, so you can feel better, and be better. Let it be your cheap form of therapy.