I just cannot seem to do both at the same time properly. It sounds very mediocre compared to when I'm singing without playing or vice versa.

Anyone else in my boat? What do I do?!

Yes, I used to be in that position. But I came across one song I was learning (Mad World-Gary Jules version) and I just automatically could sing and play at the same time. Every since then, all the other songs that I couldn't play and sing to..... I could now! It was strange, but maybe you'll find your song that just clicks. If that never happens, you just have to practice and be able to just play that song automatically and it will happen.

Yes. It takes a lot of practice because you have to remember to support your voice WHILE playing your instrument correctly.

The best way to do it well is to just play it over, and over, and over, and over, and.....Well, you get the idea. Eventually playing will become automatic and you can focus more on your vocal technique.