Why is my Cat so Needy?

I love my cat, Mel Mel, to pieces. She is roughly 5 years old, we think. We had taken her off the streets at a very young age, literally right off the side of the road, and she has been very needy since day one. Even going so far as to try and suckle off my dog (who didn't particularly mind, other than there was no milk there. They have a very strong bond).

I cannot have my lap to myself. If I am sitting down, she has to be attached to me in some way. If she's not on my lap, she's curled around my neck or trying to get inside my sweater. If I'm up and walking around, she follows me everywhere and meows at me all the time. I'm not sure why, though, she has food/water/clean litter box (not that she's very finicky about it anyway).

Even at night, she is sleeping on top of or curled around my head OR she's cuddling in my arms under the covers like a teddy bear.

I don't necessarily mind all the time, but I am a digital artist and often she tries to get between my tablet and my hands and it makes it sort of difficult. If I don't trim her claws too, I get lots of scratches because she tries to dig into my sweater or in my hair. She'll also leap long distances off counters and couches to be on my shoulders while I'm up and around, which you can imagine can be very painful if I'm wearing something that doesn't have sleeves.

What are some ways that I can make her more independent, and why is she so needy?