I agree with most of what has been said except for this:
If the pony barges downward, you just hold tight. YOu don't pull back, you just make your arms like concrete and the pony pushes herself painfully against the bit. She will bounce off the bit and you give her a small reward of a TINY bit softer rein and go on about your business.
I would say that in my experience, hardening your arms against the pony's movement would only encourage the pony to tug harder. Initiating a tug of war with any horse, no matter the size, is asking for trouble because they are so much stronger.
I would say, maintain your contact, keep your hands and arms soft, and nudge forward with your leg. If you keep the forward movement in an even rhythm, the pony is less likely to fight with you, because you're keeping her focus on the movement you're asking for rather than arguing against something she doesn't want to do.
As Strange said, the frame will come only once you've engaged the hind quarters. You can't have the former without the latter.
Remember to breathe! When you're breathing, your body is more relaxed. When you're relaxed, your seat automatically deepens and becomes more effective. With an effective seat, you're in more of a position to use your legs to drive the movement from behind. Once *that* comes, your pony should start to develop the frame. Another thing I remember my mom teaching to alot of kids, imagine your reins are sponges filled with water and with every stride you're squeezing a little of that water out of them. This keeps your hands steady, but soft, playing the bit just very gently across the tongue to keep the pony's attention focused on what you're asking. A focused brain can't misbehave.