How can i protect N-MOSFET from failing
Why this is happening and is there anything i have to improve ?

Your circuit has no fly-back protection circuit to prevent back emfs from transformer primary leakage inductance. It's crucial that some form of protection be applied if you are to avoid voltage fly-back exceeding the limit on the IRFZ44 MOSFETs (55 volts but no more).

The above is an example of a snubber that dissipates the energy from the leakage inductance.