I agree with DD, not having much of a closure could be the reason why it has been harder, also 4 months isn't really that much time. I think with beloved pets as much as with humans, the first year is the hardest.
Having lost a few pets in my life, I can say it doesn't get easier either, but a bit more bearable with time. We all grief in a different way and times, don't be hard on your self and let your emotions flow as the come.