let me point you some things.

she says he’s kind of chubby and ugly plus he has a GF so I should not worry

Here she's pointing physical attributes. Why? Because she tries to make you think there's no possibility of cheating nor developing feelings. She didn't said "i don't like how he behaves", "I don't like him as a person", "i don't feel confortable", no. She just told you he wasn't attractive enough.

so hard that apparently she started contacting her former supervisor for help on things she didn’t understand

It wasn't possible for her to find a mentor or helper in her new job? Hmmm...

she’s telling me it’s pretty clear the guy likes my girlfriend

Tries to make herself trustworthy. If she knows he likes her, why don't she stop contacting him? Even when you stated you don't feel comfortable?

I don’t like this and I tell her to stop contacting him cold, no calls no texts, she agrees. A few weeks later I learn she has this meeting with him to “break up with him as a friend”

So she agrees. Then you "learn" they bot agreed with a meeting. It's no contact so hard for her? She doesn't value your relationship it seems.

I dint know about this meeting or else I would not have let this happened.

So she lied in your face. You found out but not trough her.

So who is this guy she has feelings for? The bloody supervisor.

And you believe this feelings started after the meeting? I'd rather say she had feelings the whole time, and that's why she contacted him and lied to you.

she has NOT contacted the supervisor since the “friend break up” but that she still thinks about him

She already lied to you. Even then, she thinks about him. Hmmmmm...

a lot of it has to do that im not doing too well professionally and financially and she felt attracted to him because she felt “safe”, he recently got promoted to manager (congratufukinglations). We spend the WHOLE day together working things out (and crying together

So she also sees you as someone less worthy than him. She felt she had to tell you his possition.

and she was waiting till after my birthday to break up with me so I would not be sad on my birthday

Here you go. She wants to leave you. She had it planned.

She says she doesn’t want to be with him, she just wants to be alone so she can work out her feelings, she is confused because she loves me but she’s a little freaked out about marriage and spending the rest of our lives together when im her first and only boyfriend.

Yeah, she wants to be with him. She wants to see if other pasts are greener. She wants to fuck other people, because you're the first one and she wants new experiences.

She is sobbing and I hug her, I decide to grow a pair and channel my inner Humphrey Bogart from Casablanca (Here's Looking At You, Kid), I keep a straight face and kiss her once last time and tell her that ill always love her, and start to leave, she stops me and she thanks me for being so good to her all these years, and asks me if I think we will see each other again, I smile and say “I think so, goodbye ” and walk away, as im driving away I can see her sobbing in the drive way.

Nah, you didn't grow a pair. You told her you'll be there for her, so she can go out there, have sex and relationships with other people, and then be sure you're still there. You should have said: I think this is over, thanks for everything, goodbye. No, we won't see each other again.

I’m happy to still have her in my life, I love her completely, and again I want to marry this girl

Do you really want to marry a girl like her? She lied to you, ditched you, compared you to her crush. Well, it's your life.

just to be clear, no physical infidelity happened on both our parts

Are you sure? "She didn't cheat on me, i'm sure because she told me so"

the reason for the break ups was she felt guilty for having feelings for someone else and she dint want to hide them.

I consider the whole meeting with him at your back and lying you hidding it. I consider the whole telling you about her feelings months after them hidding it. But whatever.

I really think our relationship has a future

If you're capable of keeping this relationship dynamic sure, but she lied to you and ditched you.

im a bit angry but nobody said it would be easy and she is worth it

If you consider it that way then stay with her. Personaly i don't consider there's something worth enough to compensate being lied to and being ditched.

so I want to stay with her, or am I blindly in love and making a huge mistake?

I pointed you some things in your text. This is just the things you told us. Now think, Are you okay with them? Or you're "blind of love"? I'd say you're making a huge, very huge mistake, but it's your life.