Why can't I cum

I used to masturbate a lot and came almost every time. Then when I got in a relationship, I stopped because I was having more sex so I didn't need to. Now, my SO has been working so much that sex is put to the side. I've been masturbating and watching porn and at first, it feels good then after a while, I get frustrated and stop without finishing. I can't seem to finish anymore 🙄