I'm really happy.

We also say, "I am in my comfort zone." These two statements mean that we are completely comfortable doing what we are doing. If I am very good at mathematics, I am in my element or comfort zone doing a job that requires me to add many numbers or to calculate prices for example.

I am doing something that I really like to do / I am doing something that really suits my skills/background.

Maybe something like "It's (really) my thing" (It really makes me happy and I'm interested in it.)