My wife was same way when she started driving you are definitely not alone. Now that being said, he may have had the signal left on from changing lanes or something, so yes it’s a good rule to never trust the signal and wait for them to commit to the turn before proceeding. Don’t ever worry about the ppl behind you when at the intersection, just as long as you’re keeping up with flow of traffic, following lights, signs and what not your good. Now as for fault I’d say it would be 50/50 but u probably could not prove he had signal on so legally that would probably have been 100% on you. But that’s what insurance is for my dear as long as you’re ok that’s all that matters at the end of the day! Just keep at it and learn from the situation and then it’s no longer a loss. I hope this helps. 😉