I made the same mistake than you with my very first game. You should keep the speed of the projectile somewhere and use Pythagoras to calculate the length of the vector after the bounce.
However,Unity or C# can do the calculus for you once you understand your mistake.
Join us on the discord https://discord.gg/UkCsfVq, we have an other Jam with almost no rules (the practice circle jam) until the end of year that will be perfect to finish your little game. we already have 1 little game and two prototypes published,and it just has started 3 days ago. I encourage everyone to publish their game or prototype as soon has they want to have the help or the feedbacks of the community, they will not be contest or lockdown of the games and it will be an other jam of this kind just after if you don't have the time to publish your game. (all the month of January for the next Jam ). If you need the direct link to the Jam:https://itch.io/jam/practice-circle-jam-laboratory But a part of the feedbacks will be on the discord.