Is it a sin to commit suicide?  The short answer is yes, because God commands us not to murder, which includes murdering oneself.  The sixth of the Ten Commandments states:

Exo 20:13  You shall not murder.

However, for someone who is feeling suicidal this is of little comfort.  Here are some important facts to realise and appreciate which can help.

God loves you.

Rom 5:8  But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us.

Psa 36:7  How precious is Your lovingkindness, O God! Therefore the children of men put their trust under the shadow of Your wings.

God will help you if you ask Him.

Mat 7:7  “Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you.
Mat 7:8  For everyone who asks receives, and he who seeks finds, and to him who knocks it will be opened.

God promises not to allow life to become unbearable.  Sometimes “the way of escape” is extra strength.

1Co 10:13  No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it.