A fifth straight league defeat for next-to-bottom Bangor, who were leapfrogged in the table by the team which beat them on Sunday, Llanfair.
These sides had drawn 1-1 at Treborth back in December, but the Powys hosts finished on top in the return.
Harriet Davies put United in front in the 20th minute, but Menna Evans replied for the visitors on the half-hour.
In first half added time, 16-year-old Becky Jones restored the home lead.
Llanfair went 3-1 up five minutes into the second half when the experienced Mary Griffiths converted from the penalty spot.
Bangor were not finished, with Evans scoring her second on 52 to cut the deficit to one again.
However, the 57th minute saw Becky Jones complete a double for the hosts, who had lost their previous four league encounters.
Bangor manager Aron Griffith said: “Clearly we weren’t good enough today.
“We were second to everything and didn’t make the most of the chances we had. However, our goals were well worked and in spells we played well with the ball, but overall we were the second team today.
“Hopefully we can pick ourselves up for our final three matches to finish strong.”
Bangor still have to visit top three teams Connah’s Quay Nomads and CPDM Y Felinheli before ending the season with a trip to coastal rivals CPD Y Rhyl 1879 on March 24.
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