There IS a genetic component, and part of it is just who you are.

However, I'd suggest investigating other training methods, too. My club's coach keeps saying that to ride faster we have to ride slowlier first. And for a long time. She advises the following, and I noticed it worked for me:
80% of our training time should be at below Lactate balance point or high zone 2
10% at LBP-5
10% above LBP at threshold

To me, high zone 2 means about 155 bpm, or 70-75% of my max.

So in other words this means intervals training about once a week, two lighter recovery rides, and one longer ride at a very sustainable pace. I find it really hard to keep to low HR (and hence low speeds) like that, but the method works, and I am improving. I can now undertake long climbs and keep my HR below 160 all the way to the top, at a somewhat decent pace. Something I never dreamed of before. So when I do decide to go harder, I have room for it (HR-wise), and I can go faster. But I can't do that all the time if I want to improve. Your body improves during recovery, not during effort.

So if your riding group does not allow you to go at a pace where you can keep your HR low, maybe it's better to go with another group.

By trying to hit it hard every time to go faster, you're not improving.

I'd also suggest being patient... 18 months is very short in the life of an athlete. My boyfriend has been doing triathlons since he was 10 years old, and never stopped training. Of course he's much stronger than I am. So what?