I currently want to learn violin. I already have some background in music (but not classical). I can play piano and guitar too. So, is it difficult to learn violin by yourself even with some background in music?
I doubt that this is a good idea and it even might be an unhealthy idea, but of course the answer depends on what you want to achieve.
If you develop wrong habits they will stop you from making progress.
Violin is an especially critical example, because it´s very un-anatomical. Wrong habits in holding and using the instrument might damage your motion apparatus.
To play a note on a violin you need to hit the finger on exactly the right spot and move the bow at exactly the right speed and pressure in the correct way and at the correct position, to be able to play the next note you will to have to be able to do so fast and with the right technique and of course you need to be able to choose the right position for your finger out of the handful of possibilities.
That is to much to figure out for yourself, I think. You are likely to develop "alternative" techniques, improper shortcuts to solve the vast amount of problems and you will probably end up being able to play something but in a dead end. Further progress will be hindered by the wrong habits and shortcuts.
One obvious little example is the pinky finger. You don´t need it to play easy pieces, so folk tend to roll it in, which makes movement of the other fingers faster and easier. But of course you need the pinky for a little bit more advanced pieces or to avoid the change in timbre involved in using empty strings. But when you rolled your pinky for two years it will never ever get the flexibility it would have had when doing it right right from the start.
tl;dr You need a teacher to find a learning-path for you or your journey will come to an premature end and you won´t be able to travel to the really cool sights.
Sort of.. I have seen some Youtube videos of people who are doing just that. My son also just got a violin and spent about a month trying to learn himself (he has trumpet and some piano experience).
Simply put, the results aren't very good.
So I think that maybe it's possible, but a teacher will pick up no mistakes in technique that you are going to have a difficult time self-diagnosing.