The brain, like the rest of the body, is an organ that can be exercised. It can be strengthened, and without strengthening it can remain weak.
Those who are new to paganism are often very daunted by things as long as the Invocation of the Goddess, or even something as apparently simple as remembering the Quarter Calls (which you must remember if you're in an Alexandrian or Gardnerian coven for example!).
We can't or shouldn't fall back on simply saying our memory is crap and leave it at that. My knee is currently 'crap' because it is recovering from surgery, but if I use that as an excuse not to do my physiotherapy, I will not heal and have strong muscles.
You start small, you learn memory games (looking up "memory games" on Google will be very helpful) and read chants that you want to learn before bed. Make sure that the ceremonies you are learning are ones you want to learn, and are important to you, so that the passion remains even when you get sick of learning the same words over and over again.
As anyone who has had to learn any discipline - piano, skilled artwork etc. will tell you, you won't always enjoy it. Sometimes you'll get so sick of playing the same thing over and over again that you'll scream. But one day you'll hit a point where you play that song without thinking, your fingers know automatically where to go, and your heart can sing through the song.
That is what you are aiming for in high-ceremonial magic, and very few people get to this stage because they don't have persistence.
In the meantime, you can make smaller, less complicated rituals for yourself. But you should really be working towards training your concentration and memory skills, for the high-ceremonial stuff if that is what interests you. It CAN be learnt, and falling back on 'my memory is shit' is no excuse, in my opinion.
