Well people can be misdiagnosed for bipolar when they actually experiencing BPD. The difference between the two is bipolar would be fine but then have a episode that will last for months. High or low. bpd it will only last say emotional distress for couple of hours. It depends how severe it is.

It does sound like BPD.
I just had traits though. So for me was mood swings, uncontrollably crying. Suicidal thoughts and self harm. But I never hop from one realtionship to other. The whole extreme intensity and obsession of the person. Is part of the black and white thinking. I experience this too. It's when we put this person on a peddle stool. But then we devalued them and see them black. We struggle to see the grey. Meaning people can make mistake, do struggle but can be a nice person. I have experience the grey and it's really strange to experience but its heathly way.

The whole cheating thing. Which I have never done. But I have heard being in group therapy who experience BPD is that its fear abandonment if it's real or not. So cheating is to prevent the abandonment and the real feelings that come with abandonment..meaning feelings of shame, grief or sadness.

The self harm. I have stop for 6.months because the help I have. So there is hope for you because everyone who experience BPD goes to DBT therapy.
The self harm is to not feel the emotions. To numb it out. Because it's so painful and unbearable to feel. Also its because we dont know how to express our feelings in an effective way. So it's a cry for help. The problem with this behaviour is that people respond to this and help out and so we think oh this works. I will keep doing it. But eventually people get terid of it or it really emotionally affects them especially the ones that love you dearly.
There is ways to over come self harm which is call distress tolerance and others as well. For example if you feel really emotionally distressed putting your face in cold water. It has to be icy cold and the water has to reach just were your ear starts. Hold your breath to maybe how long you can but not till your blue in the face. Maybe 20 seconds? To This creates a driving effect which scientifically proven slows your heart rate down. Now I myself have done this and I notice you may have to do it more than once. But slowly in time I dont need to use it as often because I have built not to take others people opinions of me as who I am as a person.
The self harm isnt good and I will say why. Because as my therapist told me it's like climbing up a hill and when you self.harm you trip and it stops your emotional growth as a person. So that's why when I did self harm my emotional.growth was like and yes this is a judgment on myself but it was honestly like a teenager. Because I never learnt to feel my feelings.
Now the self harm wasnt day over night it took time. The weeks in between gradually got bigger that I wasnt self harming as often. So be patient within yourself and compassionate to yourself when over coming this.

I hope this helps.
I wish you a healing journey.