Here’s the situation: at your current point, you’re suffering from three situations here:
One is that you’re playing what is currently not a super-strong spec (and I feel you, I main Destro myself). That hurts performance.
The second is gear. A lot of what you’re suffering will resolve as you gear up.
The third is, based entirely on how you’re talking in the original post, skill.
At the low end, you would probably suffer just as much as affliction or demonology as you do with destruction…until you build up skill and gear.
At the high end, affliction is miles ahead of destruction in capability, but that’s assuming you have the gear and skill for it (and it’s not the same skill as needed for destruction). For example, I can do moderately well with destruction, and I’m not the top in the meters…and do worse with affliction, because I don’t have that muscle memory for the spec and I’d have to relearn the intricacies specific to affliction.
All specs, yes, all, even destruction warlock, can do the content that most people are interested in. Normal raids, I’m personally 10/10 CN as Destruction. Heroic raids I’m currently 2/10 CN (just started dabbling in it this week, and the raid team I’m running with is adjusting to the changes on Hungering Destroyer), but will finish before the tier is over. Mythic raids…Destro suffers…but for me, I’m not super-concerned, that’s not my forte anyhow.
The big issue that comes up with playing a “weak” spec is more one of perception. People see a destruction warlock and assume that must mean too much weakness in content that spec literally doesn’t matter for (which is up to Heroic raids, again)…or they just want a free carry (and you don’t want to be in that group anyhow).
TL;DR: Play what you enjoy. Odds are that by the time you’re at content where spec matters, the whole paradigm could shift, so enjoy the game.
P.S. You’re off to a great start with a damage meter to check your own DPS. I personally pair that with some weakauras and boss mods to help me focus in. Keep at it, and have fun!