I had a romantic interest of over two and a half years once. I ended it not because she was seeing a therapist, but because she was not (and definitely needed to). At some point, you have to detach yourself from a sinking ship if the person is not interested in doing the difficult work of saving themselves.

My opinion is that if you are doing the difficult work of overcoming long term depression, you would be best served with an ally in the process. If the person you are with is not the sort who would stand by you in this, then I'm not convinced that person is healthy for you to be seeing. Essentially, your relationship with that person is creating a barrier to your getting well. Even if it is a small barrier, it is pernicious. One should not have to tip-toe around getting better, and ideally close friends and family should have your back.

If it were me, I've got close friends and a wife who I would certainly tell. I would have told her before we got married too, but every relationship is different and I totally understand why there could be no small amount of fear involved. At the end of the day though, the closer someone is to you, the more they should have your back in situations exactly like these, in the best of worlds.

I personally don't see seeing a doctor for depression, even severe, as a big deal. Everybody goes through it at various times in their life, and if one needs some help managing it long term good on them for taking care of themselves .