Can i mix arborio and carnaroli

It's the starch that gives risotto its creamy texture. If you try to do it with regular rice with low starch there is no way you have that texture. Stirring is to prevent rice from sticking and it is not necessary (at least not constantly).

In the Do The Rice Thing episode of Good Eats, Alton Brown says:

I have to tell you, I've tinkered with risotto quite a bit, and I really don't think that that whole constant stirring thing is necessary.

Okay, here's what movement does in the pot. Now, as the kernels rub together in the hot liquid, they literally scrub off some of their starch, okay? Now hot liquid is added slowly because if it were cold, the starch wouldn't come off. And if too much gets added at once, the rice kernels would never make contact.

Now, I find that occasional stirring keeps the heat and moisture evenly distributed, but gentle pan shaking creates a better gravy.

I started not to stir (I mean constantly) the risotto after I watched that and it turns out great. However, slow addition of the liquid is still necessary.

It doesn't sound like you're doing anything wrong. I've found risotto to be one of those recipes that can vary up to 50% (usually less) from what the author calls for and still turn out great. I've even used recipes that call for "2-4" cups of broth. You may be overcooking it though, the rice should be al dente. It's not uncommon for people to overcook this until you have a mushy risotto.