You are right. Instead of having an army and a navy. I should have nothing except forts.
You said you have 3 active forts, 3 heavy ships and 18k troops, all on high maintenance. You are also at war (a mistake), so I assume you must be able to use war taxes. Mothball the heavy ships, lower maintenance on your army, mothball the forts, drop an advisor. All would allow you to support an additional few forts.
Looking at your picture, your forts your land is too scattered and your forts are badly placed. You could cover every single province you own with 3 forts, you don't need to sacrifice anything to protect your lands with forts but you have made the mistake of leaving the forts in the provinces you captured and removing the one in your capital, which is what has caused your problems.
So like I said, it is your own fault your forts don't cover the province.
You don't even know how many provinces I have. But since you mentioned it, I will tell you I have 14. The conversion gives you one. With a religious center of unity, you have 50 months to convert six more provinces to hit 50% where the disaster ticker will halt. That's because it ticks up 2% per month, but a religious center of unity takes a percent off (as does getting over 50%). However, with the loss of my religious center of unity, not only did I lose a province, and thus eight months, but I now was left with needing to get to 75% unity with a ticker of +2. Eight months for the province that had the center and 9 months for most of the others. I have five provinces converted and six months on my next one. Nine months for the one after that. That is 15 months at 2% per month which is 30 more. I already have 55% so that will be 85%. I will then need to convert four more provinces to get above 75%. At one point per month, that leaves me 15 months. Not even enough time to convert two of them. And of course, I only have about 150 ducats saved up so no defender of the faith.
So deal with it? It was your choice to convert religions, it is not supposed to be easy. You made a mistake and it makes things slightly more difficult.
Keep converting, crush the religious rebels when they hit, let the disaster kick in and then get enough religious unity for it to end. Get +1 stability from it as well. It will be annoying, but its hardly the end of the world.
Anyway, im not sure why you would ever leave Catholicism as Brittany, you have a national idea that gives +2 papal influence.