I'm sorry to say this, but I think you need to accept responsibility for the problems here.
Unless I read it wrong, you basically said that a doctor and a youth pastor convinced you that touching yourself was not a sin, so you kept doing it.
Then, when you met your girlfriend, you engaged in sexual activity before marraige.
Despite this, you claim to have:
"spent the last 10 years abstaining from sex"
Would you call 10 years of lustful masturbation "abstaining from sex"?
So you felt uncomfortable when you tried to quit masturbating and you couldn't sleep? Guess what. Every Christian guy who'se ever quit masturbating has gone through the exact same thing.
And yes, you should have stuck to it no matter how much pain it was. Steven took stones against his body for the sake of God. Paul said he treats his body like a slave (1 Corinthians 9:27) instead of giving in to it's sinful fleshly desires.
Like every person who has ever been angry at God, you don't have the right to be. He outlined the laws which would give you a happy marriage and you didn't follow them, so you can't blame him for what's happened as a result. Luckily, He is a faithful and merciful God and He can restore your relationship to 100% even after you make mistakes.
Think about it. Your girlfriend, according to you "was raised in a family where talking about sex is taboo", and she obviously cared about waiting, but instead you seem to have used your uncontrolled sex drive to lead her into sexual sin by "releasing" each other. I'm sure you both played a role, but as the man, you should have led her in purity instead.
I mean think about it. By giving into to masturbation for your whole life, you taught yourself to give in to your sexual desires. That is why you should have controlled yourself from the start, so that when you met her you would have been an expert at it and you could have waited
properly until marriage.
Oh, and just a pro tip, by demanding sex x amount of times per week, you are probably doing the exact opposite of making her want to have sex.
At first I thought her sister was in the wrong for saying:
My fiance's sister has convinced her that I'm going to cheat on her after I get married. "If a man's wife can't satisfy him, he'll just go somewhere else to get what he wants" is what she keeps telling my fiance.
But guess what? She is totally correct as you proved when you said:
I can honestly say that if we never have sex, that I will probably walk out on her
So I think instead of blaming God or your fiance or your fiance's sister, you need to look at yourself and examine your path in life and see where you have done wrong.
Then repent about it and move on.
Here's my advice for your future.. If you love this girl and you want to be with her, sex or not, then stop ALL sexual contact and masturbation until you get married. Stop even lusting over anyone. Just cut it all out cold turkey.
Tell your fiance that you have examined how you have acted and you realize you were wrong in alot of ways and that you are going to correct that. Tell her (And mean it!) that you want to be with her whether you have sex twice a day, or twice a year, because you actually love her and you want to spend the rest of your life with her no matter what.
If deep down, the Holy Spirit is telling you that your premarital sexual activity as a couple has been immoral, you should repent as a couple and decide not to do ANYTHING until marriage. Ask her if she thinks the sexual activity you two have done was sinful and if she needs to repent for it... she might be so happy to be able to unburden herself of that guilt by praying with you as a couple to repent for it, so that she can trade her heavy yoke of guilt for Jesus' light yoke.
I think if you do this and stop blaming God for the results of your own errors, you will get married and be quite happy, by the grace of God. But if you plan on "walking out on her if she doesn't have sex with me", then you don't understand that "love is not self seeking" (1 Corinthians 13:5) and you shouldn't get married.