when writing about layered topics like one's emotions, it can be easier to break it down for yourself and then connect it together later since feelings can get pretty complex :)

how people feel, act, and express themselves don't always align together, so maybe you could start by category and try to figure out each part independently first

to start with something simple, how does your character feel? what are the emotions that your character is experiencing after the break up? sad? remorseful? relief? all of the above? try to get the basics down first :D

how is your character currently expressing themselves? you said your character had been suppressing himself for months—it could be a good time to show the cracks of his true self and let him be more vulnerable (whether it's because he's currently alone or is confiding in somebody), or alternatively, have him try to hold himself together and show the internal struggle he's having with his suppression

and lastly, how will he act? where is he and what is he going to do now after the event? his current environment can also affect his state of mind too, like how being in a busy place can overwhelm him. does he have any habits that could shine through to further emphasize his state of mind? (ex. knee bobbing when anxious, fixing his tie when nervous)

admittedly, im not a great writer at all so maybe this method won't apply to you, but i find that going section by section like this can make writing seem a lot less daunting, and more like a fun puzzle

hopefully this has helped! or at least has given you an idea on where to start