So here's my situation, and how I learnt to overcome it.

Recently I discovered how to sing flawlessly. Like, with no tension at all. A lot of power. A lot of range. I was very happy with myself, because it felt like I'd finally discovered the holy grail I'd been looking for this whole time.

The only problem is that I haven't been able to replicate it since. I tried doing all sorts of things, but the issue is that ultimately it came down to a feeling, not a process - so I didn't know how to reproduce it. Anyway, after a lot of struggle and trying different things, I realised it's because I wasn't priming myself to be able to replicate good technique. And since doing this, I've been able to sing consistently.

Ultimately it comes down to the basics. Singing actually is really simple and easy, but only if you have the basics down. The reason why people struggle with singing is because they don't have all the basics down. Even if you miss a tiny piece of the puzzle, you're kind of screwed. The good news is that it's really easy to check if you are doing the right thing.

Now the issue is that a lot of people simply don't do this check. They blindly try and go into their singing thinking they've "got this", but even professionals with years of experiences do checks. Surgeons, pilots etc. Singing really is no different.

So here's a list (in order) that I focus on before I sing, which has helped me tremendously.

1. Simply focus on breathing normally through your mouth, with no tension. You want to do this with an open throat, and just spend some time ensuring that you can do this comfortably in a relaxed state. Do not move on until you have this down. Do not focus on breath support, just this one aspect - breathing through your mouth with an open throat.
2. Next step is to begin thinking about breath support, and connecting your throat to your diaphragm and learning to breathe from your diaphragm. Ensure that it's not causing you tension. One way I do this is by putting my thumb underneath my jaw. If it moves from my breathing, then I need to work on my breath support. Ensure your throat is still relaxed and open. Do not move on until you have this down.
3. Next step is to begin making vowel sounds, without tension. It doesn't matter where in your range you start, the key here is to play around and ensure you're singing without tension. If there is tension, it means either you're not engaging in steps 1/2 OR you're simply not singing correctly. That's cool, and what this step is here for! Again, put your thumb underneath your jaw and check to see if it move as you go up/down in range. If it does, try singing it again differently until your jaw doesn't move.

Now, I would argue that until you're able to do all three of these things effortlessly, then you're not ready to sing yet, or go through your exercises.

Where most people struggle with is step 3, and the reason for this is simply that they don't have proper singing technique down. In a sense, step 3 is your training. How I've described it is just what helps me, but I know there's many ways to approach that.

Personally for me, I can't do my exercises properly without completing step 3, so it's like a mini-prelude to that. Anyway, hope this helped!