First and foremost, I am sorry for the loss of your sister. It is difficult to lose a family member to suicide, especially someone with whom you were so close. It is important also that you know that your sister’s suicide is not your fault. You may feel guilty but that guilt is misplaced.
When someone is suicidal, it is a sign that they are severely suffering and likely have a serious mental illness. Most mental health professionals struggle immensely with how to appropriately deal with suicidal clients. For the layperson, knowing how to appropriately deal with a suicidal individual, even when it is a family member, is nearly impossible. Love cannot cure mental illness nor can it prevent someone from committing suicide. Family members simply do not have the education and training that is necessary to deal with the complex problem of suicide.
With regard to poor decision-making, the fact that you are aware of your problematic decisions is very insightful. Many people make poor decisions and remain completely oblivious to those mistakes.
You can’t change history but you can make better decisions in the future. No one can be free of mistakes but it is important that going forward, you recognize the importance of being correct in your thinking. To be correct in your thinking you need to think critically about every decision. Critical thinking skills can be learned.
There are two main ways to develop critical thinking skills. The first is to study the characteristics of self-actualizing individuals. Self-actualizing individuals have the ability to see reality clearly. The more accurately you can judge reality, the fewer mistakes you will make in life. It is important to make as few mistakes as possible because you will pay for all of your mistakes, financially, psychologically and so forth. Read Abraham Maslow’s work regarding the characteristics of self-actualizing individuals.
The second way to develop critical thinking skills is to enter cognitive therapy. Cognitive therapy can assist you in examining your past mistakes and the associated faulty thinking process that accompanied those mistakes. Cognitive therapy can also assist you in assessing your ability to be logical and will challenge you to make the most rational decisions.
You may have made mistakes in life but you can positively affect your future by learning critical thinking skills. You asked about whether or not you have a mental illness. Though I cannot know with certainty, I do not believe that you are mentally ill. It is most likely the case that you made decisions without utilizing the proper critical thinking skills. The good news is that these skills can be learned through research or by entering therapy. Please take care.