Interesting game, and I like the colorful aesthetics. But I found that some of the levels were a bit unnecessarily difficult. I think some could have been separated into different levels to make it a bit less frustrating. Also, would have been nice to be able to just use the mouse rather than the arrow keys. One issue I found happening was that marbles would bounce straight back from walls even when they hit at an angle.
Specifically, for the levels being too long, there was the one really long level that was winding and some parts very skinny (I think it was level 4), which after the long winding section was a totally different section that could have been its own level. This made me feel more like the long winding part was just a chore / punishment for every time I made a mistake on the second part rather than a fun challenge.
There was also the one where you essentially make a bridge but they easily bounce off so it has to be super precise (can't remember if it was 5 or 6), which felt like there should have just been one or two less slots to fill. It just felt too repetitive and was easy to make a mistake and have to start all over.
I know there's never really time for playtesting to find these things though, so overall you did a great job!