Serum not blending in?

This is a problem I haven't been able to solve for years now, and I'm comfortable enough here to share it: I use serum every morning, per all beauty recommendations, but it just doesn't seem to blend in. So I end up walking around with an oily face and oily hair that sticks to my face.

What am I doing wrong? How do I make it actually blend in? Am I using too much? (If I use any less, a single bottle of serum would last me, like, three years, so I feel like I'm doing the proper amount.)

What. Do. I. Do.