No, the Heroic campaign is not "too difficult" for the simple reason that there are two easier difficulty settings available, "Normal" and "Easy."
Therefore, if your current skill level in Halo Wars is not sufficient to beat certain missions on Heroic difficulty, you can always just play on "Normal" or "Easy."
So you have actually two options:
1.) Keep learning and training in Halo Wars, until you get good enough to beat missions on "Heroic" without problems.
2.) Lower the difficulty settings.
With regard to your concrete problem: If you build only supply pads, one reactor, one vehicle depot and one field amory, then you should be able to upgrade your defensive turrets to better turrets in the field amory. Also, do not forget to upgrade the turrets in the front of the base to flame-mortar turrets and the two turrets in the back of the base to missile-turrets. The front turrets will be good against the waves of Covenant infantry and the turrets in the back will effectively take out banshees.
Upgrade the Scorpion to the "canister shell" capability and also upgrade wolverines as much as you can. Then only build Scorpions and Wolverines. Use the Scorpions "Y-ability" (canister shells) against the Covenant infantry, especially the Hunters. The Wolverines will take out enemy air units. You should put three Wolverines and one Scorpion to the place where the first Rhino is stationed. They will shoot down Banshees and Spirits, before they can even unload their infantry on the battlefield.
Make sure to save the game from time to time! If it looks like you lose, you can just quickly load the saved game. By doing so, you will not have to start all over again.
I am currently on my first playthrough of the Halo Wars: Definitive Edition campaign and I started with "Legendary" difficulty. So far, I have not encountered any real problems. Only the mission "Arcadia City" needed a few tries, but that is mostly because how the Spartans behave and how the Covenant spawn is pretty RNG.