You have asked quite a delicate and complicated question - and I can’t pretend to account for all the subtleties of this issue in a short answer. But as a general, introductory response to the issue let me make three points.
Firstly, I would suggest that if you know of someone who is struggling with suicidal feelings, that you encourage them to seek out the help and encouragement they need to support them both at this time, and also to assist them to deal with the underlying problems that have given rise to their feelings. The apostle Paul encouraged Christians with this reminder:
"No test or temptation that comes your way is beyond the course of what others have had to face. All you need to remember is that God will never let you down; he’ll never let you be pushed past your limit; but with the temptation he will also provide the way of escape, so that you may be able to endure it."
1 Corinthians 10:13
Secondly, the Bible teaches that what determines whether a person"goes to heaven or hell" is whether they trust in Jesus as their Lord and Saviour. Jesus has done everything necessary to deliver us into the renewed creation that he has in store, and what we are called to do is entrust our lives to Christ as the one who is able to do this. What this suggests is that the issue is perhaps better considered along the lines of whether a person has committed suicide as an expression of their complete rejection of Christ, or whether it was an action arising out of a less significant weakness of character. However, having recognised this, it becomes clear that it would be impossible for any one of us to see into the heart of another person (and perhaps even our own heart) to make this kind of judgment - only God can. He is the only one who completely understands a person’s suffering & circumstances, and who is able to make a perfectly loving, just and wise determination.
Thirdly, to approach the issue from another angle, it is worth asking the question; ‘Would God want one of his followers to take such a course of action?’ To this question we can clearly answer ‘no’. Suicide (or, for that matter, euthanasia) is never a right course of action to take.
God is in control over all the circumstances of our lives, including our sickness and suffering, and, yes, even the times of our deaths. And he places us in these circumstances for a purpose - though often we cannot see what that purpose would be at the time.
God is able to use our sufferings for our good, for our growth in faith and character, and also to help others. We cannot know the whole depth of God’s wisdom; how he will use our lives to serve others, how he will use our sufferings to mould and shape us. What we do know is that God is trustworthy, and that we are called to entrust our lives to him, whatever circumstances he may lead us into.
The apostle Peter wrote to encourage Christians who were undergoing suffering, saying:
"Therefore let those who suffer according to God’s will entrust their souls to a faithful Creator while doing good.
And after you have suffered a little while, the God of all grace, who has called you to his eternal glory in Christ, will himself restore, confirm, strengthen, and establish you."
1 Peter 4:19 & 5:10