Personally, I think only He knows our hearts. Of course suicide is a sin, because it is outside His will, right? I reckon there are so many extenuating circumstances, though, that might lead one down such a path. Let's say one is a rooted in saint, fully confessed and completely trusting in the blood of Christ and in Christ now living, but he or she has a intense tribulation like Job faced but he or she loses his or her way all the way to self inflicted death. Would that be a damnable offense? Maybe, but I don't think so. I just have no way of knowing. Or what If someone is mentally ill and or broken, are they as culpable as the one who just takes their life recklessly for what seems like no reason? To the one who did such a thing would, in their minds, of course have their reasons. Again, so hard to say or to know. I do know my Lord is merciful. I think He takes their hearts into consideration.
So is it a sin, yes
An unpardonable one, that is a call I can not make. I think, like with the rest of us saints who sin, He knows who are truly His. I don't know, though. I am speculating which is something I think we here are all doing.