I think reading about enthusiastic consent is good. Sadly too many kids / teenagers / young people have little sex education and enter into early sexual experiences with lots of confused ideas. Hopefully things are changing and the idea of 'enthusiastic consent' which is always active and informed will become the norm. This is a great thing!
However, this does not mean you should hold an 11-year-old accountable to the same standards as an adult, or even yourself as a teenager. Absolutely not!

You were close in age (it's only when the age gap is over 5 years that it is automatically considered abusive) and it sounds more like exploration than abuse. I think it is impressively honest and decent of you to recall that your cousin asked whether she 'had to' and the kind of thing that many teenagers and kids wouldn't recognise as pressure. I think it's pretty understandable that 11-year-old you didn't know better.
But the great thing is that now you do! Our brains change massively over our teen years so that at a very literal level we become different human beings to who we were when younger. There is absolutely no reason why you should be shackled with guilt over something you did when you were so much younger. Since she doesn't seem affected or bothered by it in the long term I might almost say that it is good you came to a deeper understanding of consent earlier rather than later. I think it's now so, so unlikely you will ever pressure anyone in a relationship as you get older (like 'but we haven't made out in ages' or 'I'm worried you don't want to cause you don't find me attractive any more') and will be aware if anyone tries to pull that sh*t on you.
It sounds like a painful experience and time for you. But I honestly think it will be better. Tumblr in particular (if you have been reading on there) can be a really harsh, judgemental place. It has good information but a lot of people act as though if someone has done something wrong or problematic there are no longer a human being... the fact is, there are biiiiiig degrees. Taylor Swift is not Sam Pepper. You are not Josh Duggar. Truthfully, some of the most righteous people I have known have perpetrated some of the worst behaviour - a friend who get can very preachy on Tumblr who "jokingly" committed an assault; a poet who performs feminist material, but was pretty seriously emotionally abusive (amongst other things) to her ex-partner. I've talked to both these people about their behaviour (and my own behaviour) and I think as they have gotten older they have become better people. Sure, some people theoretically might never forgive them - but you can't work towards having everyone like you.
The important thing is that the victim is now okay and that you can live with yourself. I honestly think you deserve to.