Okay. So first things first, you need to ask her to stop seeing/hanging out with that coworker because that relationship is inappropriate and will be destructive to any progress that your relationship may have. Next step, work for improvement. It's not going to happen overnight, but you and your gf need to have a sit down and come up with plans to have healthy communication and fix issues that bother your partner. If you find that you guys need some help, there are relationship councilors that could.

Example: My partner has told me that he doesn't like how fast I get annoyed or angry with him. That was a personal issue that I was receiving therapy for, and told him that I'll fix it. I wasn't perfect, and apologized when I messed up. But I made a plan with him and adjusted when need be. I told him that when I asked, if he could please leave me alone for a while so I had time to cool off. I know that my short fuse and how I handle my anger is an issue in our relationship, so I remove myself until I can come back level headed. Again a lot of mistakes but after a while of going to therapy and empathizing with my partner, that time grew smaller and I'm not frequently frustrated anymore.

She thinks you spend too much time on your phone? Sit down with her and tell her your sorry for making her feel that way. Tell her you'll fix that, and both of you can come up with plans to fix that. "Hey, whenever you feel like I'm spending too much time on my phone and not enough with you, let me know. Sometimes I'm mentally tired and just want a break, so maybe you and I could have some quiet time on our phones once in a while. I don't want you to feel like I don't appreciate you, so let's set up weekly dates."

Set up clear and reasonable boundaries within your relationship. Work on yourselves as individuals, and with eachother as a couple. I didn't mean for this to come off preachy, so I'm sorry if it did. I just somewhat relate to your scenario. Best of luck.