Okay, one part of the original post that I missed or glossed over was that “he broke up with you”, and apparently laid that at “your OCD and depression”.
Frankly, on that basis – though I can understand your own sadness at the state of affairs, considering your own commitment to the relationship and plans for the future – it sounds like you dodged a bullet. From my current understanding of the timeline now, plus the details in the Reddit post and your follow-up in this thread, he made this about you, and it clearly was never about you. It was about him and whatever changes he’s going through in sexual orientation and emotional attachment, and probably a good dose of selfishness on top of that. And now he’s coming back to you for additional support and understanding.
Not a real prize.
But I realize that regardless of what kind of clay his feet are made of, that doesn’t address your own feelings of loss. That is, you’ve lost a sexual and emotional partner, an anticipated life partner AND a business partner. That is a lot to lose all at once … and then to have him continue to hang around you for additional support must be like icing on the cake.
For my own part, I regret having missed some of that (the part that should have been absorbed from the OP here), as well as the additional details that I have since learned. So I apologize for the flip tone of my second response. (I’ll stand by the first response, but that could also have had a better tone. I certainly didn’t realize the multiplicity of your loss at the time, nor the fact that he is still looking for your emotional support and understanding.)
So, advice for you, if you still want it:
He’s had time to make his own realizations and conclusions, and he broke up with you. You now need the same kind of time to grieve and get over the loss, some detachment from him (obviously!) and distance to work out a path forward. So, do whatever you have to do to realize, accept and absorb that you have had this loss, because you have suffered quite a loss, apparently. In the first place, you need time away from him, and no contact (from him, at least, especially contacts in the nature of requests for “more support” or whatever) while you come to terms with this.
Tell him to leave you alone for awhile so that you can have the time and space to come to terms with this. It’s fine if you want to be a friend and to still be his best friend, but you also need to keep in the back of your mind that this has been brewing with him – obviously – for three years (or more) and it has hit you out of the blue. This is now about you needing time to deal, to process and to get over the loss. When you’ve had time to process this more dispassionately, you may find yourself coming to a different realization as to his value as a friend, lover and life / business partner. He may not be the shit that he appears to be from my vantage point, but I’m still giving him the old side-eye.
Finally, what kind of friend would break up with you, telling you that “it’s about these issues of yours” (in the first place) and then take up immediately with another man and then come to you for your acceptance and emotional support? I would more than halfway suspect that this guy – who would be no friend of mine! – might be part of the reason behind whatever other emotional malaise you’re feeling. I would be very careful of ever letting him get within arm’s reach again.
Once again, I apologize sincerely for what I missed in your opening post, and I hope that this makes up in some small way for that. And I do hope that you’ll stick around on Fluther. I’m usually better than that.