Keeping your good periods going longer?

I'll feel really good, work out, get all my work done, not be an asshole to my friends, etc. for about a week or two weeks, but then the depression and severe mood swings will hit me again.

Can anybody recommend some mental strategies to not feeling that the dark stuff putting me back in my place won't be inevitable?

i also would like to have a suggestions

I mean, it sounds like you're focused on the right things--exercise, focusing on work, seeing friends...but I know once you start to feel bad, all of that goes out the window. My therapist keeps talking about behavioral ativation. If you can somehow force yourself to do all of these positive activities when you're feeling like crap, the idea is that your body and mind will start to feel better again.

That's all I got.

The best we can do is to keep doing what we do when we're in the good phases. Keep exercising, eating healthy, getting enough sleep, etc. even though you feel like crap. (I know that can be an impossible task some days, just do your best. like if you usually go for an hour run, walk for a few minutes around your block, or at least step out your front / back door to the outside. )

It's hard to build consistency but with practice it will happen. Are you on mood stabilizers?

i recently tried to stay in cafe for hours and grab some coffee or tea. but now i can't do that again because i found out that i'm allergy to lots of food and substance, including coffee and tea...and milk

i was just registering myself for gym. but i'm not sure i can manage the commitment

so now i used to just walk around without certain direction, enjoying bus ride, going to shopping malls or bookstores, going to cinema by myself,
or just stay in a restaurant (with limited option of food to eat because i'm in allergy therapy now)

not that i say it's helping for me...because it's not, frankly.
but at least i get myself busy than doing nothing.

The hardest thing for me is sticking to my routines, even though I know my routines help to define who I am, where I am and gives me the security I need. But like rubber, the old BPD ugliness rears its head again and tries to get me back to the original shape, so resisting it is hard work, but with time and practice, mood swings and depressive episodes get lesser and lesser.