Is it normal that i can only masturbate by fucking my mattress?

Yes, for years I almost always masturbated by humping a blue rug on the floor of my bedroom.

This came from the way I first discovered masturbation. I had belted myself to the leg of a rocking horse and I was hanging suspended in the air with the belt cinched back extremely tight into my stomach (related to a fetish of mine), and I really really liked how this felt, and I was thrashing around with my groin clipping and striking against the floor, and then suddenly... something happened. It felt really good, and I had an "event" in my pajama pants. It took me several seconds to realize what had just happened to me, and I was surprised and sort of stunned. That was how I accidentally discovered masturbation.

It wasn't until I got into college that I switched to doing it single handed.

I always masturbated the same way, by laying flat on my belly and rubbing my crotch against a soft bed or mattress. It's the most pleasurable way for me. I have tried other ways but it just doesn't work. I read online that doing it this way can lead to problems down the road though, mainly that it becomes difficult to orgasm during actual sex with a woman because that is a different kind of sensation.

I used to be like this, too. For a while, I could only masturbate by humping something. But then I kind of transitioned to being able to do it "normal". It was a little by little process, not overnight. I still to this day can't do it by grabbing it with my fist and yanking it like a lot of people seem to do. I can do it lying down on my back or side without humping if I kind of just use my fingers to stroke the underside of the shaft.

No.