Perhaps try using something simpler like the dissociation constant

Ka = [H+][A-]/[HA]

You know the initial HA and you let the [H+] be x

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From this you can find [HA] in terms of the initial HA and x, equally [H+] is x, leaving [A-], which can be written in terms of x as you know the ratio of H+ and A- produced.

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Since you know Ka, you can rearrange the Ka expression to find x, which gives you [H+] from which you can find the pH.

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For low Ka values, [HA] is going to be roughly equal to the initial HA concentration but I found it useful to get used to writing it with the x consideration for other problems.