Bullies and trolls pick up on people who appear vulnerable, who are down or have low self esteem. They target in on those individuals. So, be careful about what you say in a public forum where everyone can read everything you say.
Keep personal issues, pain offline and confide in only a few close friends - even then that information can still leak out to a wider group.
If someone starts to harass you what do you do? Learn how to be assertive online and offline. It is also just a great life skill.
There are numerous online resources:
YouTube has a range of short videos on assertive training where you can pick up some tips. Go to Youtube and put in "assertive" or " assertive training teenagers".
Here is a video on assertive training from John Cleese it is a bit old the principles are spot on today. Well lets face it who doesn't like John Cleese.
There are several videos on YouTube that can offer different tips. Watch a few and look online you will soon find useful information that will help you be more assertive.