I'm confused about why you find the relationship sexually unsatisfying. Just because she prefers that you take the initiative, or because you're the one to suggest new things, why does that make the sex unsatisfying? What do you mean by, "she has no idea about how to pleasure me"? Is she totally passive when you have sex? Are you failing to be aroused, or having difficulty maintaining an erection or achieving orgasm? In what way are you "unsatisfied"?
I've tried talking about what I like so she can do better but I can't teach her how to **** a man
What is it you think she should know about "how to **** a man"? What is it that she's not doing for you? Do you need her to be more seductive or teasing or aggressive? Who are you comparing her to? A porn movie? A fantasy?
The more I tell her about what I want the less pleasure I get from what she does as I don't think sex should be about taking instructions.
But sex can be partly about taking instructions. Everyone's needs are different, people are aroused and turned on by different things, and communicating these things to a partner--"instructing" them--is important. Is it that you are bothered by her lack of spontaneity? Do you need her to show she wants you?
Relationships are about sacrifice but I feel like it is only me who is making the sacrifices.
Apart from not experiencing anal sex, which she is entitled to decline, even if she loves you, exactly what are you sacrificing? What are you giving up in the sexual relationship?
It's making me have second thoughts about getting married because I don't really like the idea of being sexually unsatisfied for the rest of my life and that maybe she doesn't really love me because if she did surely that would motivate her to want to make me happy.
Look, I think you might be getting cold feet about the idea of marriage. Neither you nor your partner were sexually experienced before your relationship. And now you've been together for 6 years, which is longer than some people even stay married. Even though you're not married, you might be going through "the seven year itch"-- you may be considering other options, like what it would be like sexually to be with another woman, or whether to leave the relationship. Maybe some of the initial passion and excitement in your relationship has dimmed after all these years. Maybe the idea of marriage vows and "forsaking all others" is freaking you out because you've never sexually been with any others, and you're wondering what you're missing out on.
You describe a sexually responsive partner. You've said, "she enjoys everything that we do" and you say she achieves orgasm. The only thing you've mentioned she won't do is anal intercourse, and that's something many women prefer not to do because it can be painful, or for other reasons. Some women don't like to perform oral sex either. Other women are more aroused by having oral sex performed on them than they are by vaginal intercourse. Everyone is wired differently, they respond to different types of stimulation, some things turn them off, some turn them on. None of this may have anything to do with whether they love the person and it's somewhat unreasonable to demand, "If you loved me, you would...". Some people are more adventurous, or creative, or lusty than others.
Unless you can be more explicit about what you feel you're "sacrificing" in your current sexual relationship, and what needs aren't being satisfied, it's hard to tell whether you're being reasonable in what you expect in a long term sexual relationship, or how your current relationship might be realistically improved. You have a partner you say you love and with whom you want to spend the rest of your life, and your biggest complaint, that you've clearly identified ,is that she tends to be somewhat passive and depends on you to take the initiative sexually--which, all things considered, seems relatively minor, and something which shouldn't be leaving you feeling so unsatisfied or as though you are making "sacrifices". If you can be more explicit about what's missing for you, sexually, I'd find it much easier to understand your problem.
Offhand, it sounds like you and your partner might benefit from seeing a marriage counselor, preferably one who might also have training as a sex therapist. Your girlfriend may well be feeling pressured by your sexual needs and demands, and bothered by the fact that she's not living up to what you expect from her. And you feel sexually dissatisfied and as though you're sacrificing something you feel entitled to. That's not a good way to start a marriage. Those are things you could both
benefit from exploring and working out. And, if you haven't been able to work these things out by yourselves in 6 years, you need some professional help. And you haven't yet said anything in this thread that makes me feel you can't work these things out--you haven't described any insurmountable problems--particularly with the help of a good therapist.