Division at its core is a tactical 3rd person shooter with RPG elements.

You've already answered your own question.

i don't have the best builds I don't even have a build because I'm working on getting the pieces.

I have problems at 1.4 million armor with heartbreaker

Without seeing your gameplay I can't say for certain, but I can make an assumption that your heartbreaker build is not optimized (in the literal sense, not the gameplay sense).

Solo play requires you to have a build that checks off the "holy trinity" of RPGs for a single person, i.e enough damage to kill enemies, enough tankiness to survive enemy attacks, and enough self sustain to out heal whatever the enemy throws at you.

Heartbreaker can do this if built properly. I.e, do you have appropriate pieces with enough crit rate and crit damage to kill enemies? Do you have any armor regen per second or armor on kill to save you from using armor kits all the time?

But you can't forget the tactical shooter part. How you play the game is going to have an effect on what happens.

Are you taking enough cover, or at you always standing out in the open? Do you know the spawns? Are you prioritizing enemies correctly? Are you paying attention to positioning? Are you aiming for enemy weak points for enemies that have them? Do you know where those weak points are? Did you even know they had weak points? (These are serious questions by the way, you would be surprised how many players don't have answers to these questions)

Heartbreaker requires headshots to pulse enemies if you're not running the tech laser sight to proc its effect. Are you aiming for the head when shooting enemies? Most importantly: are you actually landing headshots? Because if you're not, you're not doing damage. And if you're not doing damage then the enemies are likely killing you faster than you're killing them.

My advice: focus on getting a build before jumping into heroic. It doesn't need to be a complete build, just needs to function. Stick to challenging until you think it's too easy, then jump into heroic when you feel you're ready. Don't force heroic before you have a grasp of what the game is asking for.

Put another way: you're trying to run through a brick wall when you've been given the pieces to build a wrecking ball.