It's cliche to say and a bit cheesy, but the first thing to remember is, they are still the person you know and love now they have told you or have been diagnosed with BPD, it changes nothing about the person only explains the reasonings behind their actions. 

     This is a biggie with BPD, we are mostly terrified of people leaving us, and will do anything to stop it. This could range from your usual teacher at school, to a romantic partner and anything inbetween. 

      Remember, sometimes although people with BPD are terrified of losing you, they will still sometimes try and push you away, as a pre-preemptive strike into not having to face the feelings of being rejected and so cut you off themselves - this is usually when emotions are high and they are feeling pretty irrational and struggling to cope. Don't take this personally or ask why they are suddenly being 'clingy' or 'needy', they are most likely really struggling at this moment in time and need some reassurance that you aren't going anywhere, that you are here for them and that you want to help them, so ask them to let you in. Never walk away in an argument, this will only trigger the worst in their emotions and think you'll never come back, communication is key with someone and talk your way through problems rather than storming off mid-argument. Needyness can be developed though, so it is important to recognize  when you feel like boundaries are being passed and communicating what you expect from each other when they are feeling calm and rational.

Intense Mood Swings

Another biggie, for me, my mood swings mean I can be in absolutely blissful happiness doing something, being out or sat with my boyfriend or whatever, and suddenly just burst into tears and be filled with sadness, only to be sat wondering why the hell I was ever upset a few minutes later. Obviously this can be hard to deal with when you're unsure why someones emotions are constantly changing with someone, but remember they aren't in control of this and hate it just as much as you do if not more. I am always so apologetic to whoever is around and feel guilty for bringing on this sudden mood change on them and absolutely hate myself for it, but it's out of your control. 

      One tip I was told that sticks by me is to feel the emotions. This might sound silly but it helps me to sit and feel what emotion I'm feeling, a lot of the time I just cry and have this strong sense of everything whirring around my head and makes me shake/shiver with what's going on, so i try and focus myself on what I'm feeling and figure out what has triggered it. You can help someone do this by noticing when this comes about and finding a similarity between episodes, as well as helping them through by just looking after them and asking them to talk if they want to. 

I decided to put these two together as I find they come hand in hand.. when I'm feeling unsure of who I am and what I'm doing I tend to make big life changing decisions, applying for new jobs, deciding to move out, planning a new educational adventure or something smaller like changing my hair colour and getting new tattoos. This is what I use to cope with episodes of feeling like I've lost myself and try and find an identity in changing the things about myself and changing my life. If someone around you is doing this, you can help them by discussing what they want to do with them, try and rationalise for them the decision they're making, can they afford it? Will this really make them happy? And be there for them if they hate the decision they have made afterwards.

      It can also be good to reassure someone who they are, tell them what you love about them, what you find makes them them and talking them through it. Just try and be there for them no matter what they do and understand they are struggling with an internal battle.

This ties in with the fear of abandonment for me, I struggle to not fear someone will leave me and tend to run away from people before they can leave me, after I ended a long term relationship I struggled to ever feel close to someone and would cut ties easily with people I was seeing and finding excuses not to be with people because I was so scared, you'll notice I mentioned a boyfriend so I have managed to overcome that battle but it can still be difficult, communication is key in any relationship but especially so with someone with BPD. If you are friends with someone with BPD this can also come into play, they may find it difficult to make and keep friends so as much as it may feel chore like, make sure you are dropping in on your friends, checking they're okay and aren't trying to run away from you in fear you will leave them, keep them in your life and cherish their friendship.