This is a topic many avoid discussing. Oh well... Let's talk about it anyway. Keep in mind I can only discuss things from my training and personal experience. I am not saying this is true for everyone. The role of mental health in relationships is complicated and dependent on getting the correct diagnosis.
In my 20s I was diagnosed with Depression and Anxiety. In order to treat me my doctor cycled me through the entire SSRI classification at the time and even an SNRI or two. Nothing really worked. I was either depressed or shall we say, "less than stable". I eventually stopped the medications because I didn't like how they made me feel. It wasn't until my 30s that I was re-evaluated and diagnosed as Bipolar with anxiety and PTSD. Again they tried medication which had the opposite effect. I was so tired all the time. It had taken a few years and a few Doctor changes to find what worked for me.
Why is any of this relevant? Well. in my 20s I was on medications that raised my energy level. For someone that turned out to be Bipolar this was very dangerous. On top of decreasing my inhibitions it allowed me to engage in some behaviors the current me is not too proud of. Antidepressants for someone that is bipolar can actually push them closer and closer to a manic state. This is not fun for anyone depending on the type of mania. When I got the accurate diagnosis I was put on medications to lower my energy level and minimize the chances of a manic state. This killed my interest in relationships and sex because I was too tired to care.
Now that I am in a better mental health space I can look back at the lessons I have learned and be more accepting of myself. When in a relationship with someone who is struggling with their mental health it is important to know that it is a struggle. You may not know where they are in the process of finding what works. Walking with someone that is going through this process is less like a roller coaster and more like experiencing unexpected G-Forces on a Sunday stroll. Take care of yourself too.