There are two things I can think of that may be causing your ends to thin:

The first is that ends are always thinner than the rest of our hair. As our hair is growing, hair that lives closer to the bottom of our head gets a "head start" over hair that lives higher up on our head. Because the hair higher up has a longer distance to travel to make it to the same length as hair lower down, it has to grow for a longer time before it can catch up. In the mean time, the lower hair has kept growing, so it's still ahead of the rest of your hair. This makes your ends look thin because not all your hair is there yet. This happens while your hair is still gaining length.

However, if your hair noticeably thins just as it gets to a certain length, that means it may be getting damaged somehow, and it's breaking off at the same rate it's growing in. The only way to get growth is to figure out what you may be doing to your hair that's damaging it. Once you stop damaging it, and all the damaged hair is gone, then it will grow long.

What I've found to help the most with length, is making sure I keep every bit of it that comes in. This means I do nothing that will cause any breakage (like relaxing, perming, but also flat irons, dry combing, lightening it, or treating it roughly).