When I see better violinists than me playing, their arms seem much more relaxed yet their bowing is so much more controlled. When I play, it sounds very scratchy, I frequently fall out of proper timing, I often use too little or too much bow and my playing has a harsher and more restricted tone.
When I ask people how their bowing got better, they just say that you keep practising and it just happens. But I don't know how to practise, furthermore, I have the sort of personality that can't abide gradual and unnoticeable progress, I like to be able to know that the sound is improving and going from something that sounds forced to something that sounds free and open.
There are actually bowing exercises you can do.
I am sure you probably already know this since you are playing for a while. Open bowing... and scales in front of the mirror. And some videos on this site.
Don't be too hard on yourself. I am adult beginner (old) and I am pretty hard on myself.
Good luck and keep bowing.
Vibrato Desperato.... Desperately seeking vibrato
From my observations, when the bowing is not straight it will sound scratchy or harsh and when one bows too close to bridge it will sound harsh.
There are several things you could practice to bow with straight bow:
1. practice in front of mirror and watch your bowing to check for straight bow
2. Keep your eyes on your bow where the bow hair is in contact with your strings could help you keep your bow straight,
3. imaging your elbow is the hinge, your upper arm is the wall, and your lower arm is the door, when you bow, imaging your door opening from the hinge - the upper arm steady. Some people suggested to lean your elbow against a wall and only move (opening) your lower arm to practice straight bow. Notice your upper arm movements are mainly for string changing and your lower arm movements are mainly for drawing bow across strings.
4. you could hold a toilet paper tube on your left shoulder and bow your bow to practice straight bow.
While doing all the things listed above, observe your wrist to see how it helps you get to straight bow, observer the relationship of your lower arm, elbow, and upper arm and how they work together to get the results of straight bow.
To avoid harsh sounding, imaging there are several tracks (each track is about the width of your bow hair) on your strings from the bridge to the end part of your fingerboard, so there would be about five or six tracks. The track that is closest to the bridge (touching the bridge or almost touching), would be loud and probably harsh. When you play soft you want to be on the last few tracks where the strings are near or above fingerboard. Try figure out the tone on each track. Mine is not good - very harsh - near the bridge, so I try to avoid playing on that track. Also, changing strings (use a different brand) may improve the sound/tone.
Sometimes harsh sounding could be caused by the bowing angle as well so if one has straight bowing, he/she will definitely sound better.
Our fingers have a lot to do with control and dynamic - use your index to control the volume, etc, I know all what I said above but still have to constantly remind myself and focus on it to work on my bowing techniques. Yes, it takes time to improve bowing techniques. Learning as an adult I noticed that my muscle memory is not as good as my daughter's who is learning as a child so I have a lot of problem with touching unwanted strings when I bow:(
I've only been playing 6 months or so, but the best bowing advice came this forum. It immediately improved my bowing. The advice was to go to the basement and bow as loudly as possible on the open strings! Well, that was the gist of it. To do that, I applied greater pressure and speed, but because I couldn't keep straight for a whole bow stroke like that, I practiced using half bow strokes. Once those were smooth and straight, I went to whole bow strokes, again very loud, really pulling the strings with the hair of the bow. After a week of that, I was much better. I continue to start each practice session with that. Then of course, I soften it all up and work on smoothness, keeping my wrist fluid. I think that the attempt to bow loudly actually gave me more control over the bow and eliminated the scratchiness that results from hesitant, cautious bowing.
The other technique that made a big difference for me was to pretend to be a virtuouso. I decided to just bow the way Fiddlerman does and copy him slavishly, including the faces he makes! Immediate improvement! Of course, all of this improvement falls apart when I start to play scales and songs because my attention goes to my left hand. Slowly, both hands are improving and working together much better.