How to do fix and seal a cracked retaining wall?

[Caro]

The wall has a damp problem from the garden behind it.  I am not able to redo the wall but would like to seal the wall so that I can paint it the same as the surrounding fences.  The corner has a big crack which I was going to mend myself but ended up having it mended by a handyman but has since opened up again.  I would like help on the best way to tackle these problems.  Thanks Caro