There's a lot to unpack there, but I'll take a shot.
1) You're not a beginner. Beginners improve in leaps and bounds and buckets until they reach a point. Once past that point improvement comes in thimblefuls. That's where you're at now.
2) Finesse in playing is a "depreciating skill" if not maintained. To keep what you have you have to continue to practice. Allowing your disappointment in your playing to curb your practice is counter-productive. You just have to push through. If you think skills degrade quickly on bass, you should try playing trumpet or a brass instrument where you have to make the notes with your face and keep facial strength up and coordinate that with air flow just to make notes. Youhave to practice to keep your precision, but if you're going to grow musically your technical precision must be maintained.
3) Improvement is often not related solely on technical skill, but understanding the musical nuance of what you're trying to play. If you have a deep understanding of the musicality of the music, your body will react accordingly and you will play better and improve faster. UNDERSTANDING THE MUSICALITY FOR SOME IS THE HARDEST THING TO HEAR AND GRASP. Some folks have a natural feel for it (i.e. Michael Jackson) and for others it is a learned skill (i.e. ME). Listen to recordings and try to identify the nuances of playing, which will help you reverse engineer stuff into your playing. It can be helpful to listen to more than one person playing the same thing and identify the differences in musical interpretation, tone, attack.
4) Pay more attention to what your right (plucking) hand is doing, not only technique wise, but how it contributes to the musicality of what you're playing. Work on the different ways you can pluck the strings and the resultant sound from the different plucking approaches. If I may use myself as an example, I would now consider myself primarily a bass player almost 3 years in; formerly an average guitar player and semi-professional trumpet player. It took me a long time to understand how important versatility of my right hand is and how it contributes to not only precision, but musicality.