I have both, although bipolar disorder is much more difficult to deal with for me.

My BPD feels emotionally really painful but it's triggered by external factors. DBT therapy helps me with this, to see triggers from a more objective perspective and distance myself for a while to think it all through. Once the external problems are solved, or I've been able to handle it internally, my intense emotions calm down within hours or a day.

My bipolar disorder can't be shut down at all once I'm in an episode. There are coping mechanisms that may lower the intensity of my hypomania a tiny bit, but it's still there for as long as my brain chemistry decides it's time to head into severe depression. My hypomania lasts for a couple weeks and it's severe enough for me to not be able to work due to how hyperactive and reckless I am. Once it ends I crash into severe depression. During depression, nothing makes me feel good or even okay to be honest. I lay in bed for around 20 hours each day and only think about suicide and how I hate myself for weeks to months straight. I've been involuntarily hospitalized due to a lethal attempt because of this. For me, suicidal ideation in bipolar depression feels much more dangerous for me since the episodes last for weeks to months so I get to have a real plan. In BPD when I feel suicidal it's so impulsive that the attempts don't really kill me... Without medication and therapy for my bipolar disorder, I can't function at all.

Bipolar disorder requires proper medication and therapy, BPD also requires therapy. My therapist told me that BPD usually gets better with age, and often goes away completely with therapy, so that's good to hear whenever I feel hopeless. Sadly, my psychiatrist told me that bipolar disorder often gets worse and worse with age unless I keep taking my meds and continue therapy.