Bougainvillea needs help

[NicolS]

I've been having troubles with my bougainvillea recently. A few months ago, it started showing signs of iron deficiency and I learned that periodic application of diluted liquid iron helped to get its leaves back into color. But a few weeks ago, its leaves started to curl and turned slightly yellow on the entire plant and now it looks like in the second picture, except for this guy on the back of the plant, this one branch, which looks like this (1st photo).
I do not overwater, the soil dries out fast and the plant is fertilized. I keep it on a terrace, which faces south with plenty of direct sunlight (I already got it inside, because our september/early october weather is worse than a november one here in Central Europe). I don't know what else I can do I think it maybe has to do something with roots, or some bacterial disease? I don't really know. The plant has been showing signs of distress long ago before I pried it out of the hands of my mom and tried to keep it alive (and it obviously needs to undergo some pruning).
Help please?

[NicolS]

There is absolutely nobody who can help?
The plant is quickly declining

[imafan26]

Try repotting it. Cut off the dead wood. Bougainvillea can be fussy when transplanting so try not to damage the roots.

In a smallish pot, I water bougainvillea everyday, but mine was outside. I had to keep it 4 ft off the ground to keep it from rooting through the pot. I currently have two bougainvillea (Barbara Karst) that have gone through the pot and into the ground. I may have to kill those since they are sending up 6 foot shoots.

When I repotted the bougainvillea, I would only take an inch off the roots but I aerated the root ball with a screwdriver. But, I would not recommend that unless you know where to poke. My neighbor saw how we repotted the bougainvillea and tried to do the same with their plant and killed it.

Bougainvillea has fine roots but it doesn't hold on to soil very well. When it becomes pot bound sometimes the root ball stays dry because the water just flows around it. (Hence, why I aerate mine with a screwdriver).

I would suggest before you repot that you submerge the plant in a tub or bucket of water for about 15 minutes until the bubbles stop and the roots are thoroughly wet. If your root ball is solid it should come out in one piece that pretty much looks like the pot. Since your plant is stressing. I would go for a bigger pot. Bougainvillea roots are fine and easily damaged so unless you know what you are doing, I would do minimal root work.

I don't know if it is the lighting when you took your picture but the plant looks to me like it could use more light. If it survives, gradually move it to a brighter spot or under some better lights.

Once the plant has made it past the shock phase in about 10 days. Start giving it a liquid fertilizer. Miracle grow or Peters to promote root growth. 1/4-1/2 strength every week until the plant recovers. Then I would give it a slow release like nutracote type 200 or osmocote(every 6 months) as a continuous feed and cut back the liquid fertilizer to once a month with something like miracle grow for acid loving plants (it contains micros).

[NicolS]

Yes, the light was a little off because the sun was already setting. I'm keeping her facing the south window (as in the picture). The plant has actually shed all its leaves but I'm seeing new buds everywhere.

Thank you for the advice! I will try what you suggested.