Lately I’ve been sad and I’ve felt empty, how can I fix this?

I would suggest getting counseling. Talking out loud about what you are feeling can often reveal feeling deep within you had previously been unaware of. Sadness and emptiness can be a result or accompany many other emotions or situations. I would start by understanding when you are feeling sad and what you are doing when these feeling are strongest and least strong. Try to figure out if there are any activities that alleviate theses feelings. If the feelings intensify, seeking professional help would be a good option to assure that these emotions can be got under control. Trained professionals can help you alleviate these feeling and develops ways to combat them.

One of the best things you can do is maintain a gratitude journal. At the end of the day write 5 things you are grateful about. It can be someone who has done something for you or probably a circumstance which made you smile or just a pink flower that saw swaying in the wind. Alternatively, You can take upon activities that give you pleasure, or that used to give you pleasure. Let's say a hobby like painting or participating in discussions - you can seek these out and force yourself to continue doing them for atleast 10 mins in a day. increase it if you find it helpful after a week.

Its about highs and lows. Since internet (amazingly, there was a time without internet, you know? :D), it's so easy to get a series of highs in the brain (dopamine rushes), that periods of sadness and emptiness are longer and people try to shorten them in despair. My advice is, don't try to dodge the sadness and emptiness. That's how the other side of the coin looks like, it's not bad per se. Instead of fighting it, I learned to use it, better. This period, this night period let's say, it's good for resting, for letting useless things go away, for re-purposing, for re-checking values and standards you find acceptable for you from now on. It's best time to plant seeds for next "day" time. Everyone knows how shitty we feel like, after we fight with the night time, and we stay awake, instead of resting and recharging. The same here, if people fight with night times, the following day times will be emptier and more superficial, more ADHD like, greed will develop, hate will develop, all kinds of issues follow after that. Let sadness and emptiness be there, the highs were, let the low happen, dont bother with it, and USE it to plan new futures, new highs that you would like, focus on next things you would like (or, focus on asking yourself, your inner, "what do I like?", "what do I want from now on?")