You should explore telling your story in the ways that interest you. It is better to try a difficult thing and grow from the experience, than to avoid 'offending' imaginary readers who don't exist.
However, there are some narrative 'problems' with the idea as a story for reader consumption, aside from just being a general downer.
Where is the conflict?
Stories are about an unfolding conflict that obstructs or antagonizes the MC. The story ends when that conflict is resolved. Your story idea has a conflict that already happened, which the MC cannot change.
This story will be difficult to present in a way where the reader can become invested. It suggests you would need to withhold chronological events from the reader, possibly from the MC's memory, to tell this story without giving away the ending.
The downside of telling the story in a non-linear way to entertain the reader with the illusion of an unfolding conflict, is it may distract from your intent of the MC's regret and coming to terms with the finality of her action. The reader will come to this conclusion long before the MC does, and without a twist or the ability to reverse and correct, it becomes a possible Shaggy Dog Story – a story that tells a lot of events but has no payoff.
The reader may wonder what is the point, since the protagonist can't DO anything to change, she just has to get around to 'accepting' a story specific (your) truth. This is a recipe for the reader to feel they are being preached at by the author.
You have a kind of reverse deus ex machina. The MC had agency and acted under her own will, but you (the author) are intervening with divine super-powers to say that she does not have agency and her choices were wrong. She can't 'undo' the suicide, you're just rubbing her nose in it to make her feel bad.
How does that help resolve her conflicts in real life that lead up to this decision?
Afterlives
As a minor frame challenge, another possible issue that your depiction of 'afterlife' will invariably clash with readers' religions and personal beliefs – I don't think it matters how you depict it, it will clash with someone's culture.
The commercial solution is to be as vague as possible as to the 'rules' of this afterlife, so as not to directly contradict or imply any particular belief system. This is the MC's personal afterlife – perhaps she has stalled in a transition state between life and the 'real' afterlife.
a similar, but audience-pleasing story
Consider the Capra film It's a Wonderful Life. An MC wishes he had never been born, and an 'angel' makes his wish come true (something that angels from religious mythology do not do).
Some differences to your premise: the MC is a wholesome character in a bad situation that feels piled on and unresolvable. Hope is lost and the badguy is winning – leading to the MC's crisis of faith, it's not his fault but the 'sin' is that he wants to end it all (not quite suicide, but effectively the same).
Thanks to the angel, the MC is able to view the world without him in it – for the audience these experiences are new (unfolding conflict) and we witness along with the MC creating an empathy-bond. We are invested in him solving this conflict because we are in the same conflict.
Most important, he is able to go back and correct his mistake – he is not able to erase his real-world problems, but he realizes his wish was the mistake – the one mistake he is personally responsible for.
I'm not suggesting your story take this turn, but I am pointing out the 'crowd-pleasing' narrative of allowing a likable, sympathetic MC to sink to a low and make a devastating (unforgivable) mistake –– but with the ability to learn from it, and return to face his problems.
His experience has a payoff that helps resolve his real-world crisis. No matter how awful the situation, he's just witnessed a world that was much worse –and he returns to face the situation but reaffirmed in the beliefs and values he wants to live by.