Retrogrades are a normal and, of course, perfectly natural occurrence. At almost all moments of the year, one or more planets are in retrograde. Mercury has simply been in the pop culture spotlight because of how strongly it impacts technology, communication and travel. Of course, these things we always notice in our modern daily lives.
Mercury Retrograde has zero negative impact on love spells, BUT since Mercury rules communication, technology and travel, it may slow down communication between you and your desired lover. In other words, you may have to wait couple of extra days to hear from the target of your love spells. Or, an email from your target will end up in your spam folder and you won't realize it until a week later.
Backfiring is a myth. What is real is poor planning and poor judgement.
The spell did not backfire. The curse simply wasn't thought through and was a bad decision.
Take 100% responsibility for the work that you do. Thoroughly think through the (potential) outcome and how it will directly and indirectly affect you. Not because of any false "laws" (karma, 3-fold law, etc.), but because you shouldn't shoot yourself in the foot.
Love & Magick,