From the recipe itself "Note: Adding excess acid makes the Paneer softer". Possibly you aren't waiting long enough between acid additions to see the result and you end up adding to much lemon juice trying to get the whey seperation. Another major reason for weak curds and soft cheese is the milk. UHT milk is really bad for any kind of cheese and theres no legal requirement to label the milk as ultr heat treated so maybe try a different brand. Calcium chloride can help with that, but it could screw up this recipe, i've never made paneer so i can't say. Also your curds look really really small which would support too much acid or too much heat and as far as i could tell that recipe doesn't even mention what temp to heat the milk to.