I am on tapentadol 300mg and oral morph sulphate 90mg for some reason i sometimes feel like i for get to breath when im nodding off, can this be fatal?

Possibly: The most dangerous side effect of narcotic medication for pain is the decreased drive to breath. It's real & depends on total dose/day. Taking enough narcotics to be "nodding off" when you are not trying to sleep is also dangerous. The 2 problems may mean too much narcotics - please speak with the doctor prescribing the medications about your side effects asap! if morph is as needed, reduce it.

Slowly or quickly!: Opiates reduce the drive to breathe. During sleep such reductions cause low o2 in our blood and high co2. Over years this can lead to irreversible lung damage, but in short run it will interfere with your daytime thinking.And reacting, which could be fatal quickly behind the wheel. This potential problem can be readily evaluated in your home overnight.