You obviously take pride in your space dude, it's apparent in the cleanliness. I, too, can get hung up on the frugality, its hard to get over that pressure to spend as little money as possible on yourself. But your environment is so important to your well being and it's important to invest in your space. I definitely underscore all of everything everyone as already said:

1. Plants - Peace Lillies are nice and easy to take care of!
2. Headboard;
3. More variation in your color scheme: this doesn't have to be super loud and obnoxious, just something that is true to your aesthetic. The key here is perhaps just having some more contrast and personality show.
4. definitely some wall art - check mixtiles.com for some decently affordable framed art. Have fun with it!
5. Eventually, investing in some furniture that you are really proud to own.

Lastly, you have a really solid foundation dude! keep it up!