OP, i have several serissas and I almost didn't recognize your tree because the trunk has not lived outdoors so it's so clean. Rain, wind and other natural elements cause the trunk and branches to discolor but the tree can be cleaned with a toothbrush. Also, it appears you haven't been clipping the tips of the branches to shorten the internodes and reduce the size of the leaves. You need to do that!
Your tree appears on the verge of death to me. How long has it lived in the windowsill? How long have you had it? This is a tree that likes sunlight. Please invest in a grow light. Don't blast it right away but it needs additional light desperately. Put the grow light on it and start slowly and work your way up. I've never used a grow light but many here have and I would ask them for advice after you get one.
Also, come spring it needs to be potted in bonsai soil. It's time to take it out of potting soil. It's not bad that's it's been in potting soil but it's plenty big enough to repot in bonsai substrate. You can buy a bag on many sites online and a grow light asap. Don't repot until spring!
Slow down on watering this plant in fall and winter.