I happen to have a Serissa Foetida. They are as strong willed as they are picky. They normally bounce back, but that won't last forever.
You shouldn't immerse your bonsai every 2-3 days, especially if it's not outside. Thats wayyyy too much water for the bugger. Although Serissa's like humid weather (they're a tropical plant by nature) you should water when necessary. I'd say that you should gently use a fingernail, or something small to scrape away the top of the soil about 1/4 inch down, if the dirt is still damp, there is no need to water, but once it gets to that point, water it.
Only use immersion once every couple weeks, to rid the dirt of stale air and dry spots. Take the bonsai over to a sink, and take temped water (not artificially softened) thats unhealthy. Rain water is best, but tap will do as long as your tap water isn't heavily chlorinated. If it is, (like if your in the city) then put water in a quart cup or clear container, and let it sit on that window sil for a few hours, and alot of the chlorine will evaporate. When watering, if you have something with a shower nossil, that is best, but if you pour really gently that will do as well. Pour water over the soil very carefully untill it's about to overfill (depending how course your dirt is, this will happen fast, or the water might drain right through) allow the water to soak through, and repeat 2 or three times until you've poured in every corner and the bottom of the pot has just about quit dripping.
You also need more sun. Serissa's need sunny / humid / weather. So if you have your heat, or AC on full blast, thats not helping either. I keep a humidifier right next to my tree, and I still have to come here and ask why it gets finicky sometimes.
I suggest you get Bonsai: A Care Manual (Paperback) by Colin Lewis It helped me alot. I can sort of tell you're grasping for straws listening to anyone thats had something green stay alive on a shelf for a while. Bonsai is a different breed of animal, and needs special attention. Just make sure you don't kill it with your love.
On a ending note I'll quote.
https://www.bonsaiweb.com/care/faq/serissa.html wrote: If the grower doesn't panic (and many Serissas are inadvertantly killed by panicing beginners who respond by drastically over/underwatering the plant) the plant will soon regrow its leaves. A power struggle then ensues. The grower soon discovers that the Serissa drops its leaves if it has too much water. Too little water. If it's too hot or too cold. If it's moved, or there is a shift in lighting or temperature. If you cough near it or use a vulgarity in its presence. If it doesn't like your looks.
Until someone develops Serissa subliminal motivation tapes, (the calming sound of waves upon the shore as voices whisper, "You are lush; you are green; you want to keep your leaves...") the ugly truth is that some growers develop a knack for the foibles of the Serissa, while others are doomed to wait hand and foot upon a plant which looks like it's auditioning for a role in "Les Miserables." However, because the plant is so lovely, widely available, and popular as a traditional bonsai, most of us try our hands at it at one time or another. So for the masochist in all of us, here is the care information for Serissa!