Long story in short:

Girlfriend of 2.5 years doesn't want to take the pill, this argument has been going on for 1.5 years and each time she tells me a different excuse.

She's 18 and she refuses to get the fact her parents know she's not a virgin. She also refuses to talk with them about sex, so if anything, she has to hide the birth control pills from her parents.

The other thing she tells me is that she doesn't want to put anything in her body that can harm her. I already told her that all that those pills are 1,000,000,000 times healthier than all that junk food she eats, but she keeps telling me it will affect her fertility (WRONG WRONG WRONG).

Lastly, she says she's "just 18" and want to keep her "innocence". Shit, we've had sex hundreds of times already, you asked me to cum inside you condomless (didn't do it) and pop your cherry (did it), the fuck did you say about innocence?

I told her I can take care of everything, I just hate the rubber and want to feel her insides. She said the same thing, but all of those reasons are what's stopping her.

I really hate the fact I have to "convince" her to do it, after all - we both get pissed when the condoms run out and get freaked out when my cum touches her crotch somehow.

A lot of stretchmark-havin', open mouthed vagina havin', baby cryin in the other room-havin', broke the whole rest of your life havin' teenage girls have said the exact same things as your girl.

She WILL get pregnant, it will happen eventually unless she grows a brain. I'd basically threaten to leave her if she doesn't comply. I can't imagine being subject to the absurd opinions of some teenage girl when it comes to something that could effect my financial life for the next 18 years. Don't let some little girl run your life. She doesn't run shit, and tell her that.

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn

Point blank, if she refuses to use birth-control steps, and your refuse to be a "Baby's Daddy". YOU must decide the outcome of the relationship, luck only lasts so long Bro!

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They call me the cat whisperer, cause I know exactly what the pussy needs.

You need to respect her wish against putting something in her body that she is uncomfortable with. How would you feel about taking male birth control that makes you temporarily infertile? I think those exist. Makes more sense to unload a gun than to wear a bulletproof vest, doesn't it?

If only such a thing would exist, and only in case it would be temporary - I'd even prefer that. This way, I will be the one responsible.

I'm experiencing a similar problem, although with a new girlfriend, and she's already tried it once. I don't think you can convince her to take the pill, but I think others can. You just have to convince her to consider it. Firstly, you should decide how badly you want this. Are you willing to stop having sex with her to force her hand? Are you willing to break up over this? Tell her how important it is to you, and tell her what will happen if she continues to be unwilling to consider taking birth control pills. Secondly, she should talk to her parents. Point out to her that her parents are not retards, they know their daughter is sexually mature and has been with the same guy for 2.5 years. You've probably been on vacation together, you've stayed over at her place, she's stayed over at your place, etc. They know, they've done the same thing. Once the parents obstacle is out of the way, all that remains is the supposed health issue. I don't know anything about that, but I know doctors know about it, so let resolve the whole health thing with her physician. Once you get her this far though, I think she'll have changed her mind and is willing to try it.

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One of the most useful things you will ever learn about body language.

Even if you would convince her, would you trust her to use it as prescripted? A girl that is reluctant to take something that might affect her health, she may not take a new one when she vomits/gets ill or take other precautions. Birth control pills are only to some extent safe when used exactly as prescribed - which requires motivation. She might not tell you when she missed one. I would only trust a girl using the pill if the girl herself is very motivated to take them. If you want to lose the condoms, consider a Mirena over birth control pills or a different fool proof option. Then she also doesn't have to hide stuff from her parents.

I suggested the Mirena to my girlfriend and she was willing to consider it. Thank you for the suggestion

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One of the most useful things you will ever learn about body language.

I'm guessing if she won't take the pill she won't take this... but what about the implant?
Small needle jab/insertion thing for 3 years no hass.

And yeh, don't force her to do anything she doesn't want of course!
But be clear where you draw the line too.