There is no indication that damnation is a punishment for suicide, anywhere in the Bible or within any realm of competent theology. That's a weak, and unsophisticated argument. There were many, many other problems I have with what you said, but this was the first and one of the most glaring. We will explore why.
All sins are forgivable other than denial of the Holy Spirit, which is the sole suggestion of the limits of God's grace in the Bible. Even then, we do not know the limits of God's grace. To the extent you claim that suicide mandates damnation, you are not only assuming that there are limits but that you know those limits; and, the limits which you know are drawn at suicide. You will find exactly no support for that proposition, anywhere in the Bible. To the extent that any verse can be misconstrued to suggest that, there are other more probable interpretations and there are other verses which would contradict that interpretation.
Theologically, there are obvious irreconcilabilities with that proposition. Here are two examples that illustrate those, from the perspective of agency:
First, consider for a moment the implications, as well, of the proposition that suicide mandates damnation. What are the additional circumstances that control? Does it only mandate damnation if the suicide occurred after one reached the age of accountability, whatever that was, for them? How could you know that such a person would reach that age or had reached that age by the time they'd committed suicide?
Second, consider a suicide committed as a result of physiologically observable chemical imbalances in the brain due either to clinical or major depressive disorder; or, alternatively, poorly treated depression where someone was given something like Zoloft or Lexapro and that gave them the will to end it all. Is something outside the scope of your control something sufficient to merit damnation? Hardly, even if you buy into the theory of free will.
In the big picture, original sin and human fallenness is the cause that is independently sufficient to justify damnation; which, as the scriptures make clear, is well within the bounds of God's grace to forgive; as is any action taken in life, which is secondary to original sin. In that suicide is an act taken in life secondary to original sin, even if the intent was to end life itself, that too is by definition within the known bounds of God's grace.
So, it turns out that there is a range of God's grace that is known. Anything within the realm of that is forgivable, and suicide falls well within that scope. But, there is not a reason to assume that the known bounds are the same thing as the limits of God's grace. But, if something is well within the known bounds, then it must also be within whatever limits known or unknown may but do not necessarily exist.
In this way, for you to assume that suicide falls outside both the known bounds and unknown limits is beyond preposterous. Now, I also know that the idea that suicide mandates damnation is not originally your idea; because invariably you heard it from someone else, whose theological hubris and dogmatism was likely so palpable it was unknown to them (much as water is unown to a fish, because the water is all they know). So, I'm not saying you're a bad person or a heretic or anything like that... despite my tone. That is because I don't give you credit for this idea which I know is not yours. You probably heard it from like... idk... some priest or firebrand minister or something.