Is my life with horses just not meant to be?

So, since you have a timeframe, go ahead and ask the barn owner/manager/trainer about Herbie. Say, "I'm going to be able to loan a horse in May, and I'd really love to loan Herbie if he will be available". Even if they haven't really thought about loaning him yet, this will put the idea in their head, and they just might have him ready for you by May. If not, they will know you'll be ready in May and can be working on lining up another horse for you to loan by then.