I know that isn't true. I'm very confused about this. Suicide is a sin. Thoughts of despair which fail to trust in God are sin. Not every sin, however, destroys faith.
Some sins are sins of weakness.
Suicide may well be an expression of unbelief and despair as it was in the cases of Judas and Saul. In other cases suicide might be a result of a sin of weakness or of mental disorder, which at least in part has physical causes. We cannot judge what thoughts and motives may lie hidden in the heart or see if faith still survives there. Only God can. We should, however, properly have a horror and dread of suicide, because it does cut off the time and opportunity for repentance.