  We provide strong evidence of the spin-nematic state in a paradigmatic
ferro-antiferromagnetic $J_1$--$J_2$ model using analytical and density-matrix
renormalization group methods. In zero field, the attraction of spin-flip pairs
leads to a first-order transition and no nematic state, while pair-repulsion at
larger $J_2$ stabilizes the nematic phase in a narrow region near the
pair-condensation field. A devil's staircase of multi-pair condensates is
conjectured for weak pair-attraction. A suppression of the spin-flip gap by
many-body effects leads to an order-of-magnitude contraction of the nematic
phase compared to na\"ive expectations. The proposed phase diagram should be
broadly valid.
