If you're looking into getting a second horse and she fits things you're interested in, considering your horse gets along with her, i don't see what the drawback is? You already have a riding horse so waiting for her to be broken doesn't seem too bad. Can you afford to give her the proper training if in any case you can't do it yourself?
I think the problem is that the OP's current horse is on the verge of being, or already, too small for her. So, she needs a bigger horse to ride.
OP, I vote no on the yearling. I bought my gelding as a barely-handled 2yo and we started him as soon as I got him good on the ground (about two months after I bought him). I couldn't really do much because he was still so young, even though he was already 15hh and 1100lbs as a 2yo. That means it was a good year or two before I had a horse I could do what I wanted on. By that time, I had issues of my own (panic attacks when I go to mount...long story), so I wasn't ready to ride him. Now, he's a green 7yo who is sweet as can be, but doesn't have much on him in the way of training.
Think about it logically. You're too big for your current pony. You buy the yearling and you'll be waiting another MINIMUM year before you ride her, probably closer to two or three. If you want to compete, you're going to want to wait even longer before putting that kind of training on her, so it'll be another four plus years before you're really back doing what you want to do. In the meantime, you're still paying for her feed, board, farrier, vet, etc. All that time, money, and effort could go into an older horse who is already broke and doing what you want to do.
Don't take the easy way out just because you're afraid of a little effort.