Hi OP. Firstly, be kind to yourself. You are doing an amazing job. BPD is a fucker of a condition and to be doing all you are doing is amazing.
Secondly - see if you can get in touch with your HV for a postnatal mental health nurse. I had one until my DS was 1 year old. I had hideous anxiety afterwards and was allocated one because I was likely to get PND (I did) and honestly, just having her there helped. It's not too late and whatever you decide with your current pregnancy, you need that support network there. I too had absolutely fucking awful experience with MH services but for some reason, peri and post natal MH care was a lot better, perhaps because of the fact a child was involved.
Thirdly - terminations are tough but they are recoverable from. Take your time - your pregnancy is clearly in the early stages and whilst the further you go, the harder you may feel about terminating, your options are there. If you feel it would have a really detrimental impact on your MH to have another child, please know it's for the right reasons. You being stable and happy is of utmost importance irrespective of whether you have one or two or eighty children. How you balance that is up to you. Marie Stopes have a counsellor you can talk to for as many sessions as you need and they at no point push you to make a decision either way, despite being attached to the clinic. Please do not feel guilty for doing something that considers both you and your child's health.
DP wise - I stand by the fact that I feel that a decision of this magnitude should always consider the other parties choice but not be limited to if that makes sense. He cannot force you to have a termination anymore than he can force you to be pregnant against your will. But, do try and consider your feelings first and foremost - you'll be pregnant, not he, and you will inevitably be responsible for the childcare of both children should you progress with the pregnancy.
I had a small scare when DS was 5 months old too - I think it was a faulty test and a weird period but I know my mind was in a total tailspin and I didn't know what to do. DP didn't want anymore (and has since had vasectomy) and I was in the throes of PND. It was a tough time and it was either a fault test or I had a chemical (never worked out which) and I cannot imagine how it feels for you with BPD on top of what you're experiencing.
We are here for you to talk it out. Whatever decision you make we will be here 