That must be so hard to experience that feeling. You mention that you 'keep trying'. I'm curious about this. What experiences have you had so far? What have you tried so far and how has it gone?
I'm hesitant to give advice or suggestions, but I do have some questions that might be worth thinking about. I'd ask myself these if I was in your place:
- Does my potential partner need to have the same level of interest as me in zoo sex or could they be, for example, uninterested in it for themselves but unconditionally accepting of it for me? Or, maybe a lower or even higher interest in it than me?
- Is there a timeline that I decide to share the zooey aspect of myself with my dates? In other words, am I looking to share about it right off the bat, or wait a few dates and then bring it up when there is some trust established and a little emotional investment? What am I comfortable with, and how am I comfortable going about it?
- Instead of focusing on who or what I want, is there a way to focus on how I want to feel (seen, heard, touched, at peace, etc) and let that guide me?
- Is there a way to meet potential partners in the world in addition to dating sites? Where could I find people who are open to thinking outside the norm?
- Is there a way to accept that I simply don't know if there's someone who is a good fit or not, and when/where I'll find them, or they me, rather than trying to accept a conclusion that is impossible to have all the facts for and that is rather self-defeating ("probably never find someone")?
- Is there a way to treat myself like my most important partner?
- Is there a way to find low-stakes friends in the world in close proximity to me, and start practicing there in terms of talking about zoosex as a topic, not necessarily implicating myself or my personal life in it to build up some resilience?
Speaking for myself: the reality is, I think, having the expectation that my potential partner match my interest in zoo sex is an utterly massive one. It is so big that I think the enjoyment of dating collapses underneath it. What if - I work on finding someone who I think I can trust with revealing this about me after thoroughly vetting them in some different ways first? What if I put the focus on finding a holistic relationship that can hold this part of me - rather than making the relationship about this topic?
disclaimer: I have yet to find my own partner(s) and your mileage may vary!!! But this is where I have arrived at in my journey so far and I plan on giving a go when I'm ready. I'm *very* curious about people's thoughts on this as I am considering this general point as well.
I'm happy to be with my dog no matter what, would be happy to share him with a like-minded woman to, but there must be something more than only sex, if I will add a woman to me and my boy 
his life, we need to share a life together, meaning trust, honesty and loyalty need to be there too, if that's not possible I rather stay with only my dog the rest of my life in peace.