If you cannot see well enough to thread your yarn directly into a big eye needle, and have a hard time pinching the end to make it fit into the eye, you can make simple needle threaders using a piece of thin but stiff plastic, such as those that come in product packaging.
This page contains the following solutions.
If you love to sew, but your eyesight makes threading a needle, put a piece of white paper behind the needle. The contrast will make it much easier to thread the needle.
My grandmother couldn't see too well, but she was always hand sewing something. I suggest that when you visit a legally blind person who likes to sew, help them out by putting several needles on a spool of thread.
If you're having trouble threading needles, buy some with slots at the top. Just insert thread from the top. The thread won't come out unless you tug on it.
Cut your sewing thread at an angle to thread it through the hole easier.
One way to easily thread a darning needle with a large eye is to fold a thin, rectangular sliver of paper in half the long way, sandwich the end of the thread into it as you would a hot dog in a bun, then slide paper and thread through the eye of the needle.
When starting a large hand sewing or needlework project, thread several needles when you begin. Then you won't have to stop and keep threading needles as you work.
If you have trouble threading your sewing needles you are not alone. This page contains tips about using a dental floss threader to thread needles.