I have the same problem now. I don't know if my bf is definitely bipolar, I just think he probably is. He 'slow dumped' me this summer - like yours kind of 'slow dumped' you where they just withdraw and withdraw little by little or do seemingly small rude things, like cancelling or changing plans at the last minute, not enough to cause a huge row, just little acts of disrespect really.
But people with mental health issues can still be mean and they can still make decisions of their own. My take on it is, he must have known the effect of all this on me and the eventual, curt, nasty message telling me he needed space and I must 'respect his privacy' and not message because 'I ain't replying!' They know the effect because they knew how to draw you in and court you in the beginning. They know what being nasty and dismissive is just as well as the rest of us do. Me and my ex resumed convo but he was purely focused on my dog - no happy Christmas for me, just 'what did you get the dog for Xmas? Is she having a good time? Have you treated her/taken her out?' Which was insulting to say the least. It had fairly recently become long distance as he'd had to move for work, then he was back, saw friends (not too depressed or busy for that!), switched his profile pic on several platforms to smilier more engaging ones but was 'too busy' to meet up with me. I sent a message saying i couldn't do the 'casual chat' any more as it was leaving me sad, anxious and confused - true and it has made me physically ill with heart problems, loss of appetite, loss of sleep.
Fizzling is the worst ending - slow dumping is considered the most harmful way to end something after ghosting - because it denies the importance of that relationship to you both. It implies that this was a nothing ...that can be slowly ignored and that person - a real flesh and blood person - will 'get the message' and disappear. What is the message these wretches want you to receive? That you are worthless? Don't accept their message! Block, delete, erase all trace of them. Put it down to deception. I'm sorry for you and for all of us misfortune enough to give them the time of day. X