I was driving along a main road signaling left mistakenly , a car pulled out from a junction turning right , I t boned him was it my fault

No. Your signal was obviously a mistake, but the person turning from the side road must yield to traffic on the main thoroughfare regardless. If you signal a turn you are not required to turn. However, I trust you've learned a lesson that if you signal and don't take the turn you need to be extra defensive as many drivers will assume that you intend to take that turn.

Many people forget to turn their signals off when they don't automatically go off. But Its hard to say how the insurance company will rule because I have seen them rule totally opposite of what they Should too many times. They Should have control of their car to not assume you will turn, but you say you t boned them. Thus you hit them.

I'm an investigator - I say you are partially at fault. What State so I know how the insurance company will settle.

In a civil law suit, if the other party fights it, or with the insurance companies, since you admit that the signal was on. It can be used to put partial fault on you. The other driver, can state he was watching, and you were signaling turn, and he expected you to turn.

In accidents, fault does not have to be one persons, but can be a percent, perhaps 80 one driver and 20 the other, ( or other percents)

So while it is really the other drivers "fault" in court or in insurance settlement, I would expect that you will have some fault given you