# Why is my sinusodial fit not working well for this data?

I'm trying to find the best fit line for this data below but no matter what I try, the fit line seems to never be able to account for the lower values as shown below.

The x-values are just dates from 1/1/2014 to 7/20/2015 (566 values), but I don't know how to give you guys the y-values. I have it in my Environment but I don't know how to give you that without copying and pasting from the Console output.

This is the code that I'm using to get that fit line:

wb.loglik=function(theta,y,x,null=NA)
{
a=theta[1]
b=theta[2]
c=theta[3]
d=theta[4]

if(!is.na(null))
{
d=null
}

s2=theta[5]
n=length(y)
return((-n/2)*log(s2)-1/(2*s2)*sum((y-(a+b*cos(2*pi*((x-c)/d))))^2))
}
result=optim(par=c(mean(wbbcf),sd(wbbcf),1,365.25,var(wbbcf)/2),
fn=wb.loglik,x=Time,y=wbbcf,control=list(fnscale=-1))
theta=result$par theta value=result$value
value


This is the code to get the plot above:

plot(date,wbbcf,xlim=c(as.Date("2014-01-01"),as.Date("2015-07-
20")),ylim=range(c(-3.2,0)),xlab="Date (1/1/2014 to
7/20/2015)",ylab="Total Net with Storage (bcf)",main="Total Burn with
Model")
par(new=T)
curve(-0.9740582-0.7857229*cos(2*pi*(x-5.9582996)/385.1581090),1,566,
ylim=range(c(-3.2,0)),col="blue",xlab="",ylab="",xaxt='n',yaxt='n')


What else can I do to generate a better fit line for this data? Also, if there's a good way to predict future data, I would appreciate help with that as well.

Sorry in advance, if I'm not giving enough information. Please feel free to ask for any information you need and I will promptly edit the post.

EDIT: I have added the work I did with ARIMA below.

I inputted the following code and got the following results:

forecast::auto.arima(wbbcf)
fit.arima = arima(wbbcf,order=c(0,1,2))
pred.arima = predict(fit.arima,n.ahead=500)
plot(wbbcf,xlim=c(1,800),ylim=range(c(-3.2,1)))