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I think it's hard for a lot of people to create emotionally and sexually meaningful relationships.
Unless you've had out of the ordinary incidents like being slapped, punched and so on, it's probably a lack of opportunities combined with some bad luck and maybe a lack of understanding how relationships can be "hacked" a bit to help them along.

My theory is that the way we form relationships is a combination of innate ability (1) and learned behaviour (2).

(1) is common to each human and includes things such as interpreting facial expressions, sense of touch, etc, even though I agree that conditions such as autism can have a significant negative impact here.
Still, you said you are failing, which means you are trying and you do express a desire to succeed, so I am guessing that this is not what's preventing you from succeeding, but rather it's something related to your beliefs and behaviours. Which leads me to...

(2) this behaviour is normally learned as we grow up, as a combination of advice from parents, your social circle and simply experimenting. But if one has an unusual childhood, parents that teach them "wrong" things, is e.g. bullied or simply has bad luck, they might end up either not knowing what to do to form a romantic relationship or even doing the opposite of what they should do. In which case they have to learn by themselves.
Now I don't know the particulars of your situation, but have you read available literature about how human sexuality and relationships work? e.g: Pittman's Man Enough, Miller's Mating Mind, Meston's Why Women Have Sex.
This might be a starting point in better understanding what's happening and why you find things difficult.