The question is discussed in the poskim including the CC Hilchos Shemiras Halashon 6:4. It is beyond the scope and the wrong forum to discuss this online so ask your LOR
One thing is certain however. Not only is venting Loshon Hora online not permitted doing so has a terribly destructive personal effect on the people who do so.
Say the question in the OP of “So, ideally, how is a Jew supposed to express their negative feelings in a healthy way?” were asked in a non-Jewish venue as “ideally, how is a someone supposed to express their negative feelings in a healthy way?” constructive people would not answer “start venting to other people all about some of the world’s problems that you have come across”