I can't enjoy sex emotionally or physically. It never(/rarely) feels good and my mind is always dark. Yet I still care about the person & want it to work. Is this messed up?

Not at all! Sex is not a necessary requirement for a relationship to work and be meaningful. If you want, you can try to dig deeper and find out if your reaction to sex has an origin that can be explained, maybe with the help of a therapist. Or you might even be asexual, which means that you simply don't experience sexual attraction. Either way, there's nothing wrong with not wanting sex, although social stereotypes might make it seem like a romantic relationship has to include sex. It doesn't, it's all about your desires and the agreements you make with your partner. If it's something important for you, don't compromise, remember that there are people out there who can understand and are willing to accommodate your inclination.

I have a very similar issue to yours, and for me it came down to this: "Sex is not a service to give, I do not exist to fulfill, sex does not define me or my relationships." You do not owe it to anyone to have sex, if a relationship will fall apart without sex it wouldn't have lasted anyway, and if the person you are trying to please is aware of the pain sex causes you and continues to ask for it anyway they are not the type of person you want to keep around, nor do they deserve you. Your issue might lie deeper than sex, in your personal self worth. Remember you are worth more than your body and you have control over it. Talk to your partner, tell them how sex affects you, if they are unable/ unwilling to help you through that mental state or give up sex to help you, I personally don't believe you will be compatible in the long run.

Of course not! You're feelings are completely valid! Have you ever heard of asexuality? That may be what you are feeling. If you're unfamiliar with the term, it describes a partial or even total lack of sexual attraction towards others OR a lack of interest in sex as a whole. As with every umbrella-sexuality, asexuality is a spectrum. What we think of as "normal" would be allosexuality, where someone definitely experiences sexual attraction. A partial, or lack of an interest in sex, would be graysexual. There's many more terms in between if you look more in depth. For example, I am cupiosexual. I experience very limited sexual attraction, but am still open to and interested in sex. I have a good friend who is demi-sexual, and they only ever feel that attraction after forming a bond with the person in question. There are many other terms and if you take a deeper look, you may find one that suits you! If you still want the relationship to work, I'd advise you to have a conversation with your partner. Assure them that's it's not their fault or the result of anything they've done (and the same applies to you), and just explain to them how you feel. Some people may have a hard time accepting that, but a good partner would be supportive and not want you to feel uncomfortable! Don't be afraid to set boundaries and tell them what you are and are not comfortable with. There are many other ways to bond with your partner, so don't be afraid to explore! There are plenty of online communities you can research from and they are SO accepting and patient. Asexuality is extremely common; you aren't alone and your feelings are valid. :) Have a great rest of your day!