I Love Someone I Shouldn't

i met a girl a few weeks before dating my partner, we kissed, cuddled, and had a great time, i knocked it on its head because of her age (same as my sister) and went with my current partner, weeks later, my partner cheated on me, and i went straight back to this girl, we had sex, and were fine, the pressure got to great so i told my partner, admitted all, and stopped taking to her. i still fancy this girl like mad, but me and partner have a baby, so it could never be. what do i do? im sick of suffering in silence