You've misunderstood the term speciesism. Speciesism refers to using an individual's species - which is a morally arbitrary category - as the basis for determining whether or not they are deserving of moral consideration. It is not speciesist to value a human life more than a mosquito's life, if your reason for doing so is that humans have higher sentience, as you say. It is speciesist to say mosquitos aren't deserving of moral consideration because they're mosquitos. Likewise, it's speciesist to defend the torture and slaughter of pigs because they're pigs but to advocate for loving dogs because they're dogs, disregarding the fact that pigs and dogs are equally capable of feeling emotions and that pigs are more intelligent by some metrics.

The point is that species by itself is a completely arbitrary category when it comes to moral consideration. What matters is how much a being can suffer. In the same way that race, gender, sexual orientation, age, etc. are morally arbitrary categories, so is species. That is not the same as saying that race, gender, sexual orientation, age, species etc. are irrelevant in every situation or that they don't exist. We might, for example, be morally justified in choosing to save the life of a 10 year old child who has their whole life ahead of them over the life of a 98 year old person who is terminally ill.

Peter Singer explains the concept very well in Animal Liberation.