Just trying to echo all the people above me, and add a couple of things.
I struggled with head down trim for a while as well. I wanted to be absolutely perfect - be able to just hold completely still and have literally no movement head/feet/left/right. That led to soooo many dives experimenting with trim weights in various areas and trying to get things exactly right. The thing that made the biggest difference though was loosening the harness. It was like a Eureka moment when I finally did it. People tend to wear it wayyy too tight (even though you don't feel like it's tight on you) and it causes your first stages to sit too high on your back and tip you. Even lowering them 2 inches down your back by lengthening your straps will make a HUGE difference. You can "dive the gear not let the gear dive you" all you want but if you don't have your tanks sitting in the right spot for your center of gravity (everyone is different) no matter what you do you won't be able to get your trim right. Once you get it perfect though, no matter what deco tanks you add or what reels/lights/etc you add, you should be able to use your posture to dive the gear.
A few more things:
1. If you need trim weights, that's normal (I have to use a 4lb tail to get things perfect) but you need to be trimming out your kit without the deco tanks until it is perfect. Like Aj said, what are you going to do when your tanks get low or you have to take them off? Your kit needs to be trimmed out itself perfectly before adding anything else. Then when you add your deco tanks, do not add/move around any trim weight or you are going to be screwed as the deco tanks lose gas or when you need to hang them on a line or leave them somewhere temporarily while on the bottom (which is not unusual at all). Also, if you're having a head down issue, why do you have trim weight up by the valves at all?
2. Get neg buoyant fins if you're having this issue. Yeah jet's are expensive but they rock and help in these situations. Of note, you don't need neg fins to use your posture/legs out to help. extending your legs shifts your overall center of gravity even without neg fins, neg fins just make more of a difference (imagine lengthening a seesaw)
3. If you're still having issues when you add your deco tanks, move your shoulder D ring a little bit down (do not move the waist d ring however or you will make your tank sit too vertical). Moving the first stages of the deco tanks more towards the middle of the chest will help with this problem. You'll be sacrificing deco tank trim to do this but in my opinion if you can't get it to work higher up it's worth it.
4. probably already have this, but a doughnut rather than horseshoe wing would make things easier so you're not getting air trapped on one side or the other here and having to focus on moving air to the other side to trim the tanks out.
Hopefully all this helps. It takes tons of dives and time of just tweaking things ever so slightly over and over until you get it exactly right for you. You really need to be able to use your posture though once you add things so if you don't have the right fins/gear to do that, you need to get it.
You absolutely should be able to trim out two Al 80's on the left. Once your kit is perfect, hand and feet position is going to make a huge difference. Stretch those arms out and lift the hands up a little if you have dry gloves so the air in them helps to keep you from tipping, and extend your legs back to help counter the heavy first stages up front.
