Will writing about things like suicide, self-harm, and tragedy give me negative results when my stories are read and is it best just to avoid these subjects all together?
I would recommend to be very wary of the topic of suicide.
There appears to be a lot of evidence that suicide. like other behaviours, can be contagious.
As a result of this many countries have codes of practice regarding reporting and sensationalism of suicide, particularly in children, teens and young adults.
While most of the literature I have seen relates to journalism, there are certainly sources which include fiction as being part of the same effect.
Given the substantial evidence for suicide contagion, a recommended suicide prevention strategy involves educating reporters, editors, and film and television producers about contagion in order to yield media stories that minimize harm.
There doesn't seem to be much literature about the effects of suicide in fictional writing. It might be that you don't get the single 'big hit' of exposure to a population that you do with news or TV drama stories, or perhaps the different uses of 'literature' and 'novel' are confounding my search.
Anyway, my point was not to say 'don't write about suicide', but to say 'consider the potential effects on readers of writing about suicide'. And consider the effect on yourself. You presumably wish your writing to have impact and be memorable. Writing can be powerful, so writers have a duty to give consideration to how they wield that power.
Shakespeare is well-known for his tragedies, multiple of which include or touch on the concept of suicide. Romeo and Juliet both commit suicide. Hamlet struggles with suicide and has an entire soliloquy about it.
Speaking of suicide and self-harm in tragedies is far older than even Shakespeare, however. In Oedipus Rex (speaking of taboo subjects!), the eponymous character drives stakes through his eyes when he realizes what he's done.
One important thing is to know your audience. When you're going to see a tragedy, you know it's going to be, well, tragic, and that it may concern itself with such things. The cultural mores of the time will influence what your audience is comfortable with.
The other important element is to handle it in a careful way. One method that has been forever employed is euphemism - the subject matter is touched upon rather than explicitly spelled out, but consequences are explored. The novel Flowers in the Attic is a relatively modern story that involves an evil grandmother who locks her nieces and nephews up in her attic in lieu of raising them. Incestuous rape is involved, but it approaches it in the manner of examining what would happen if people who happen to be related are forced into such a situation. It is presented as tragic and emotionally damaging for all involved parties. Granted, the book is controversial and got its share of critical panning, but it has also been assigned reading in high schools at times. (And banned at other times, just to show you that mores are finicky.)
I will note that this doesn't mean the work doesn't have to be entirely serious despite the gravity. Dark comedy is a thing, though I will note that it is very difficult to pull off and falls flat for some people. In the movie MASH (and I believe this scene was in the novel? Someone may want to double-check me), their dentist, Painless, was suicidal. They held him a mock funeral with a mock suicide, in a darkly comic way. (After all, Suicide is Painless). The point of dark comedy is that it is a method of coping with these dark emotions, often by facing them headlong and laughing in their face. But it is not for everyone, and is even harder to pull off.
So no, it's not bad that you touch on macabre subjects. But you must be able to do it well, and explaining how to do it well is beyond the scope of a single post.