Great question.
From my time during the interviews, I noticed a LOT of applicants didn't have direct research experience in the areas in which they wanted to work. I didn't even have any direct research experience at all (other than familiarity with research methods and APA style developed through work in college); I worked at an MCO for a year and a half and that was my "research" background, so I didn't work with populations directly at all.
I think what's more important is that you're specific about what you're interested in, and that you show that you can have good ideas about it. Having experience to draw from is important in that regard, but it doesn't have to be experience that goes on your CV. If you know someone who has BPD (or borderline tendencies) or has been suicidal, that connection can help you come up with different ways at looking at the problem and help you generate good ideas for research. That said, you're probably best served to let those ideas guide you in your application, but not explicitly mention personal (meaning non-professional) connections with mental illness; you don't want to make it look like you're saying "I'm an expert in suicidality because I had a friend who was suicidal." And if you don't have personal connections, try to learn as much as you can about your area of interest. Read as much as you can. Watch movies or documentaries that really capture the behavior and the mindset of people with personality disorders or suicidal behaviors. That way, you can form your own understanding or theories about their behavior, and have some good ideas about what sort of research you might want to do. One strategy I used in my personal statement was to cite a study by the POI I wanted to work with and say an idea that I had for expanding that, or how it related to an idea for a study that I would come up with.
I know I've made it sound a little bit like you need to have your dissertation topic in mind when you apply to grad school; that's really not the case. You just need to be able to show that you know enough about the disorders/behaviors of interest and are passionate about it to be creative (meaning you can come up with at least one idea for a study around the topic if you're prompted by the POI). If you can do that without having research experience that's specific to that population, I don't think that will hold you back with very many professors, and will generally make you seem like a very good match.
It's worth noting that, in my own application process, I got five interviews out of ten clinical Ph.D. applications. I got into two programs, and I think those were the ones where I was the most engaged in discussing new research ideas with the POIs. I generally tailored my application towards identifying populations with anxiety disorders (GAD and trauma) who responded poorly towards CBT and modifying treatments to better serve those populations, but for one of the programs that accepted me, the POI I applied to work with was focusing on gender roles and sexuality (including sexual assault). During the interview with her, despite not having read much academic on the subject (and not doing any clinical work related to it), I just had all of these cool ideas for research. A lot of them just came up on the spot when she was talking about her own research. I didn't need any specific experience; I'd spent so much time thinking about the topic in my personal life and with the people I know that I was able to show that I'd have good ideas if I worked in her lab, despite not having any specific work experience.
Sorry this is super long; I hope it's helpful to you in your application process. Good luck!