First, decide whether you want to be a professional musician OR a song writer. The two aren't mutually exclusive but are completely different. Most of the "great musicians" have never written songs (or if they have, we don't know about it).
Obviously to write songs you need some musical ability (which you appear to have) and then it's down mostly to hard work. Most successful song writers spend years just writing rubbish which they just throw away (get rid of all the cliches and copies and dross) and even then their "failure rate" is very high. Some people seem to think that a song writer sits down, starts at the beginning of a song and works through to the end and has a completed song. It's just not like that!
Lyrically, expressing yourself is fine IF you have something to express that you can express. Otherwise, fake it, learn your craft. Write about anything - love songs, rhyme "moon" with "June", try writing some extra verses for songs you like (if that works, then try writing a new tune). Do ANYTHING that results in a song of some kind. Doesn't matter if it's bad. The next one might be better, or one of the ones you write next year might be better. Just do it. There's no secret.