Katie.....yes, you do have a good heart. You're doing a wonderful thing.
However......... I really do feel sorry for your fiance. You made him a promise, then broke it. You may want to re-visit this issue and re-negotiate your promise. If I were him, your "word" wouldn't be worth much to me. It would make me wonder what other promises you intend to break. Do you know what I mean?
You say you "found" an animal on Craigslist. Did you go looking for it?
If your passion for animals is worth more to you than keeping your word, your sense of honesty and/or your integrity, then your fiance needs to know this. If you can't keep your word for him, then this may be a deal breaker for him. He deserves better......and so do you. Maybe you just need to admit that you tried to change, but you just can't do it. Honesty is a wonderful thing, and you'll feel so much better!
Please do not take any of this the wrong way; I do not intend to be mean-spirited at all.