Trigger warnings are for public situations, not necessarily for private situations.
If I am reading a book and it has something uncomfortable in it, I can skip over the pages. I can put the book down. I can walk away and come back to it later. If the author is insensitive, or portrays the incident moronically – it doesn't matter if I personally identify with the content, or if the content is racially offensive or mishandles a mental illness for example, I can put the book down and choose whether or not it is worth my time to continue. I can also go online to find reviews and analysis that may or may not agree with my take, and I can read their advice on whether the book is worth finishing. It's my choice how to approach, or leave, the material.
However, in a public setting such as a classroom discussion where I will be graded and judged by my participation, there are real life consequences to walking away from an uncomfortable discussion. To make the issue worse, subjects like rape, incest, race killings, murder, suicide, and mental illness are often discussed (as here) in terms of literary metaphor, contrived plot devices, and simplistic character motivators. I saw Black film students protest a professor's insistence that Birth of a Nation is a great work of innovative film, because he failed to admit the objective fact that it was created to sow racism among white viewers. When they brought this to the attention of the class and received pushback from the older white male professor, their choice was to be silent or leave. Most found this Hobson's choice unacceptable. It wasn't the teacher whose grades suffered by walking away.
The difference between books/media/online discussions and real life
Here at WritingSE, when an ignorant man defends rapists because "the woman probably did not fight hard enough", it's reasonable to assume this man is an idiot, has never bothered to research his opinion, and has an arrogant Dunning-Kruger idea of a situation with which he has no experience. (Wow, someone is wrong on the internet.) Like reading a book with offensive content, I can choose to walk away. I can leave the website. Or, I can satisfy myself by adding an html-link to the actual definition, without personally engaging in an argument where one side is denying reality. I can simply post a relevant article (which I have done) and he will (no surprise) insult me to defend his bad opinion, dig in his heels, and become obnoxious until he burns himself out. It doesn't touch me. I can object to nonsense and defend reality without being "triggered".
In a public situation, the same ignorant jerk will take extra steps to defend his "right to offend" with his own made-up definition of rape which does not agree with logic or law, but he will "dig in his heels" all the same to make sure that the woman who challenged his authority is punished, and he will engage in bullying behavior to attempt to rally other dim-witted trolls to aid him. From a teacher's point-of-view, it is not about sensitivity for the experience of one student who may have suffered sexual assault, the problem is that they can no longer have a discussion about rape as an abstract metaphor in ancient Greek plays or whatever. The student who has actual real life experience should not have to suffer ivory tower academic abstractification just to suit a metaphorical discussion. Reality and real life experience trump metaphor every time, except now there are added power-dynamics, my academic score, potentially an entire semester wasted (or leave a job, or…).
The reason the term "trigger warning" entered the public discussion is because real life trolls who make up their own self-serving definitions of rape wanted to humiliate strawman assault victims by claiming university is too "soft" and coddles delicate students (as opposed to the Klingon deathmatch fighting arena they imagine it should be). The irony is that this position is anti-intellectual anyway and is the antithesis of educational discussion. A college literature class does not benefit from anti-intellectualism, and these trolls probably never went to uni anyway (or they went to business or law school). The media attention on trigger warnings was never about assault victims. It was always about claiming that universities are bad.
You don't need a trigger warning for your book.
There are plenty of literary methods you should be using to signal the contents of the story. The author can foreshadow events or use symbolism. The story will have an overall tone, and characters who lend themselves to serious subjects. There may be minor character who experiences an issue before the main character does, or there may be clues and messages throughout the book. The story's setting may be an obvious indicator – it's expected that a war story will see our protagonist experience painful and unfair truths about humanity.
Like every other important element in your narrative, you will build up to the important events, you will have characters who visibly show signs of distress and danger, and will need to deal with the aftermath to build their character arc. A narrative structure where important events are signaled to the reader is intrinsic to the art of writing.
You wouldn't warn readers that a clue to a murder mystery is about to occur (so skip ahead a few pages if you do not want to learn who the killer is). That's not how we read or write stories. Presumably your MC experiences a personal trauma, assault, or tragedy and it changes them. If it's important to the character then it is part of the story – but if you declare parts of the novel to be gratuitous and skippable, it's hard to imagine how the reader is expected to take the event seriously.
As NofP said in a comment, if you have experience with the subject you either wouldn't be asking this question or probably wouldn't be writing about it at all. Thank you for not indulging in Dunning-Kruger arrogance. Write the best story you can. If the traumatic scenes are unnecessary, maybe they don't belong in the story and should only be referred to or left ambiguous. A very strong psychological reaction is to suppress trauma. It might help to see this as less of a ticking character timebomb, and more as a coping mechanism to move on with your life – that alone is a good reason the MC wouldn't care to talk about it, or review the incident in any gory detail.