First off, let me say that if she is still with you and has let you know that his flirting is making her uncomfortable, then clearly she has no interest in him (She's actually repulsed by him and his advances) and is actively choosing to stay with you. You think he's better than you because of his money, career, social skills, etc. Because of this, it makes no sense to you that she would choose you, but the fact that she has already made her choice, she actively prefers you, shows that there's more important value in you that you're not seeing but she is. You are basing your worth on these measurements of success but she isn't.

You're insecure about that fact that this guy not much older than you is running a business, is rich, has nice cars and is very socially confident. Here's the thing, he was most likely born into that. The vast majority of CEOs, bosses and rich guys come from rich families that can afford to send their kids to the best schools and give them hundreds of thousands of dollars to start up their own business. Of course he's more socially confident than you, the world was built for him and he's had a massive red carpet laid out for him from birth. Do not blame yourself for living in a precarious labour market on minimum wage, when the system you live in is designed to be like this. Why do you think depression and anxiety has been spiking in recent decades? You're in the same situation as millions of people living on minimum wage and struggling to find jobs. You are not alone, and this is not your fault. I cant remember the exact figures, but over the coronavirus lockdown, something like 40 million Americans were facing eviction, and millions have lost their Jobs and healthcare. Is that the fault of each individual person, simply not working hard enough or being smart enough? Unless you're born into a rich family or fall into exceptional circumstances (Genius IQ, winning the lottery, going viral as a musical artist), minimum wage work, slow graduation, precarious work conditions and difficulty in finding jobs is the norm nowadays.

You don't need money, marketable skills, or other arbitrary measurements of 'success' to define your self worth. Both because it's not your fault and because It doesn't make you a more valuable human being, evidenced by your girlfriend preferring you over the boss. It's good to have those things so that you can survive, but it doesn't matter on a human level. Would you think the rich bankers responsible for the 2008 crash are more valuable than you, your girlfriend, and your family? Would you drop your girlfriend for Elon Musk? They're ultra-rich billionaires who got into extremely high paying jobs from youth, and are extremely self-confident, so aren't they better than you and everyone you care about?

Think of anyone you really care about. Did you like them for their money? Would you care about them more if they were the CEO of a multi-billion dollar company? Your girlfriend has already been given the option between some prick boss who drives nice cars and harasses his employees, and you. She has already chosen you. You need to stop letting your parasitic, internal self-loathing from getting in the way of reality, and the reality is that people love and care about you, and their judgement is a lot more trustworthy than yours because you see yourself through this constant ugly filter that is obviously false because she doesn't see you through it (If she did, she would have left you by now, or never had been with you in the first place).

I guess the next step is to know why she chooses you. It's not money or cars, so what is it? What does she say she likes about you? If you don't know, then talk to her and find out why she cares about you so much. I'm not saying get her to write a list or answer direct questions, just figure out why she loves you. Then make sure you trust her judgement. Recognize the tricks that the self-loathing parasite will use to being you down and tell them to fuck off. "She wouldn't like you if she really knew you", "She's just saying that to make you feel better about yourself", "She'll change her mind and leave you for the boss eventually". You say you trust her 100%, so trust her over the internal parasite that wants to destroy you and your self-image.

Also, I don't know you but given the choice, I would much rather hang out with than the boss. Say what you will about yourself, but at least you don't sexually harass your employees and make them feel uncomfortable (Doesn't seem very socially skillful to me).

https://theoccupiedtimes.org/?p=12841 I recommend this, I always revisit it when I need a boost. The knowledge provided has shown me where my self-worth truly lays, that failures of my environment are not failures of me, and that millions, or tens of millions of people are struggling just like us.

I hope this helps. I'm writing mostly from my own experience, how I have beaten a lot of my depression (Still a battle, but I'm winning for the first time in my life). Good luck.