There's a couple of things you can do to make this beatmap in particular have a higher difficulty.
1. Try to incorporate the beat into the vocals more (bass, claps, etc.). Playing the song on .25 speed may help you do this more effectively.
2. As Germanrekk said, spacing can also add difficulty to your beatmap, after watching it, your flow is pretty good, try to space it out so the player has to reach for some of notes.
3. You have some breaks where I think more can be added. Even if it isn't as fast as the pacing you have for the full beatmap, adding more notes could make it have a higher difficulty (or lower). This point is up to you.
4. Finally, I think some small jumps or more burst would be great for the map, try putting small jumps in places where the singer is spitting some bars, and add burst where the beat ramps up.
Not all songs are meant to be high difficulty, I think the song your using could be a five star at least. I hope this helps, if you have any questions let me know
