Hi PurpleCat23, welcome to the group

you're landing in a good place to share your experience with others who definitely understand what you're going through.
You H's diagnosis is pretty new, so it's no wonder that things have been unstable lately. How did he receive the diagnosis (accepting of it, in denial, other)? And who gave the diagnosis (therapist, doctor, another professional)?
Glad to hear you're seeing a T. Many of us here are, too; mine (as far as I know) doesn't have specific experience with BPD, but she understands family dynamics enough to see where I'm coming from and offer support.
I think you're on to something here:
I feel like I'm going crazy trying to please him all the time to avoid the arguments that come anyway.
One of the challenging things about BPD is that often there isn't a rational correspondence between what's going on "on the outside" and how the pwBPD feels.
For example, for a "generally normal" person, we might have a bad thing happen to us like a fender bender, and then after that happens, we have feelings about it (anxiety, frustration, fear, anger, etc).
For pwBPD, one way to think about what's going on is that first they have inner feelings come up -- anger, frustration, rage, fear -- and then, even though nothing happened "on the outside", they look for something "on the outside" to pin those feelings on. So, there isn't necessarily a correlation between what you did or didn't do or say, and how the pwBPD feels. This might be what's going on with you and your H, that you're noticing -- no matter what you do or say or don't do or don't say, he feels how he feels inside, and seeks something or someone to pin those feelings on.
Maybe it can be a relief to you to know that your task isn't finding ways to make sure he doesn't get angry -- in fact, you really don't have any control over that.
What we can do here is learn some non-intuitive tools and skills that we'll have 100% control over, that can help "turn down the heat" in interactions and can help us stay stable and grounded, no matter what the pwBPD is doing, saying, or feeling.
One good place to start that learning process is with our article on
What Does it Take to Be in a Relationship with a pwBPD. Check it out when you have time, and let us know your thoughts on it.
We'll be here for you;
kells76