Feeling hopeless and pointless about life

Hi. So my anxiety has been fine these days and was able to lower it to 25mg zoloft and have been on that dose for 5 months but I am encountering a new problem that I've been kind of having for quite some time now. I've been feeling hopeless and I feel as if my life is pointless. Nothing really entertains me. I do have a very caring girlfriend and family(mother is divorced though) and I am currently working as a seasonal gardener, which was one of the things I always wanted to do in my career; to work outside with nature and maintain beautiful flower gardens. I also have future plans after this seasonal gardening job, which is to go back to school to earn a degree/certificate in horticulture. Everything sounds good in my life so far right? Well, for some reason I'm just not enjoying life. I don't enjoy work as much as I thought I would and the weekends feel extra short that I feel like I didn't get to spend enough time with my girlfriend. I just feel as if everything I'm doing is pointless and won't go anywhere with it. I don't understand why I'm feeling like this, I feel very nihilistic. Is this depression? Or some other mental illness that I have?