The Catholic position on suicide has been fairly consistent, and it is laid out in an encyclical by John Paul II--Evangelium Vitae--written in 1995. We are dealing with roughly two thousand years of history here, so I will be focusing on the 20th century teaching (where I have more expertise) and avoiding much discussion of the evolution of Catholic teaching on the issue. Quoting from the Bible, Augustine, Thomas Aquinas, earlier Church documents like this text from the Vatican's doctrinal institution, as well as excerpts from Vatican II, the pontiff lays out the reasoning on the Church's position in section 66 (though reading 64-67 would give more context, and reading the entire document would give even more as many topics reinforce each other in John Paul II's writings).

Suicide is always as morally objectionable as murder. The Church's tradition has always rejected it as a gravely evil choice. Even though a certain psychological, cultural and social conditioning may induce a person to carry out an action which so radically contradicts the innate inclination to life, thus lessening or removing subjective responsibility, suicide, when viewed objectively, is a gravely immoral act. In fact, it involves the rejection of love of self and the renunciation of the obligation of justice and charity towards one's neighbour, towards the communities to which one belongs, and towards society as a whole. In its deepest reality, suicide represents a rejection of God's absolute sovereignty over life and death, as proclaimed in the prayer of the ancient sage of Israel: "You have power over life and death; you lead men down to the gates of Hades and back again" (Wis 16:13; cf. Tob 13:2)

So, while suicide is always considered a serious matter, there is a recognition that there are often circumstances that mitigate the responsibility of those who make that choice. Mental illness is given particular attention. However, suicide is still strongly discouraged as it is seen as a rejection of the gift of life and a rejection of God's control over that life.

Sacrificing one's life for a noble cause is specifically exempted from this category, as stated in the other document listed above.

However, one must clearly distinguish suicide from that sacrifice of one's life whereby for a higher cause, such as God's glory, the salvation of souls or the service of one's brethren, a person offers his or her own life or puts it in danger

For more resources, footnotes 83-87 of Evangelium Vitae would be a good starting place, but I didn't want to overwhelm you. I hope that answers your question, but followup questions from OP and others are always encouraged.

---

Disclaimer: In no way is this post an attempt to proselytize. I merely desire to answer the question directly, giving what the Catholic Church has taught on the subject. The merits of the Church's position could be debated elsewhere. I simply wish to state the history of the teaching on this issue.