When you say sex with your current partner isn’t like what you remember, I am not sure what you mean. Based on your follow-up question about not being into him, I am venturing to believe you mean that your sex life isn’t living up to prior expectations created by previous partners.
Sex is different with all partners. I refer to sex with someone new as being like a snowflake: no two partners are ever the same. Even if more than one partner does the exact same thing, the results won’t turn out the same. The context between you and each partner is different. Perhaps the physiological components are different as well, which can contribute to a unique sexual experience.
One big difference I am noticing right away is that you are considering marriage with this current partner. I am guessing that is something new, which suggests that he is unique to your previous partners. The way you feel about him, think about him, and see him in your life are all different. All of those differences affect the way you view him, and the two of you as a couple.
Sex with emotions and real intimacy takes on new context. You are, potentially, more involved on more levels. This can create anxiety for some, sexually speaking. I see women all the time who are not as comfortable sexually when there is added intimacy in the relationship. These women begin to fear how they are seen, what the man is experiencing, is it good, etc. The brain takes over and you miss the sex itself because you are too busy thinking about the sex in a disconnected way.
Try reconnecting to yourself and your physical sensations during sex play with your guy. I borrow from mindfulness practice when working with my clients around these issues. If you must be in your head during sex, then at least direct where your thoughts go and stay tuned in to what you are doing instead of observing. Pay close attention to how you experience everything you and your partner do during shared intimacy and see if that helps.
If tuning all the way into your sexual experiences isn’t reconnecting you with pleasure, talk to your partner about it. I’m not saying let him know that he's falling short compared to past partners. That is a surefire path to hurt feelings. However, you could tell him that because this relationship is so important, you are experiencing more anxiety around the intensity of the intimacy. See if there are ways the two of you could explore sharing pleasure in ways that give you a chance to show him what you like in the ways you know you like it. It is more than OK to be an expert on your own body and sexual desires. And you have a right to them, as well as sharing them with someone that means so much to you.