First of all, I want to strongly disagree
with everyone who said: "Well if he gets mad about it, then don't talk to him about it." Relationship 101 --> communication.
If you can't talk about certain subjects without one of you shutting down, or one of you start yelling, etc then you've got bigger issues than if you can afford horse tack or not.
My hubby and I have been together for over 8 years (actually married only since June), but throughout those 8 years there was one time that I stormed out of the room upset (which really doesn't count because alcohol was involved on both parts and emotions just run rampad then) and one time when our voices got a little heated. That's it. Two times in over 8 years. Why? Because we are communicating constantly. There's not one single issue or one single scenario we don't talk it out and reach a decision together.
You two need to do the same.
Are you going to continue paying more than your share of the rent/board? Are you going to help him with his debt (since he used to take care of your horse)? Or, are you not? You two need to talk about it and figure that out.
How do I get the point across that I will do what i'm going to do regardless of his opinion, because I want to and I can?
I am not a counseler or relationship expert by any means, but this statement bothers me. I don't know anything about you two but this single statement would make be very leery about you two tying the knot.
If you want to go do what you want on your terms when you want, then break it off and be single. Otherwise, there is another person in your life attached to you that you need to be considerate about. The two of you need to talk and figure this out. And if you can't talk and figure things out, well ......