Can it have that quick an effect? I'm not denying that you had a drop in sex drive, I'm just wondering if the implant can really have that quick an effect, especially as they ask you to wait a week before relying on it because your body hasn't adjusted yet.
I say if it's the best option for you (and it must be as you've had it put in), leave it for a month or so. I have the implant too, I absolutely despise it and am switching to IUD soon, I have mental health issues and did express worry about the implant worsening them before having it put in but I went ahead anyway because it suited me best, then surprise surprise when I started getting a lot stressier and more irritable, they told me they were reluctant to take it out until I had had it in for three months because my body had to adjust! This was also alongside other very annoying symptoms like bleeding constantly which obviously affects work. They gave me the "mini-pill" to fix that but it doesn't do an excellent job. I had this implant fitted in September and I am STILL bleeding and irritable, hence my decision to switch.
So basically, depending on how sympathetic your sex clinic is, they might try to encourage you to wait a bit longer. I do agree you should probably wait just in case, but don't wait as long as I did!
If you decide to switch, have a chat at your sex clinic and make it known that mood changes and sex drive are your top priority, they will give you info on the options that are the least likely to alter those things.