How can I fix this? Help me save my relationship.

Get some therapy and show him you are working on fixing this problem. Seriously. He is not only your relationship partner, but your business partner too, and this is how you treat him? You've been very disrespectful of him and you need to understand why and how to change it with some outside help. If you are lucky, he will remain patient while you do this (so long as you make demonstrable improvements). But that is only if you are lucky. If you are unlucky, you still need to get this sorted out to find out why you would continually do this to someone whom you supposedly love.

-what can I do to make it up to him? To make him feel better. To make him feel that I’ve grasped that this is the end of the line and I want to make this better and not ever screw anything up again. To make him feel I will make a sacrifice on my part that for once puts him first. He's been telling you want he wants and you *know* what he wants. He wants you to keep your promises. He wants consistency. He wants you to be present in both mind and body. He wants the safety net of knowing if he doesn't do something, you will take care of it, that the entire burden of all the scutwork that is part and parcel of daily living doesn't fall solely upon his shoulders. He wants to be an adult in an equal partnership working together as a team, not someone in a relationship with a child in an adult's body. I've been in this relationship you have, on the other side, and I'll tell you that it's just soul destroying. Fix it or have the good grace to go away.

But I’m bad at romance and bad at planning. I don't have any life-changing ideas, but I want to head you off at the pass here: a big rom-com display of affection WILL NOT HELP. It will make things worse. I promise. In terms of concrete steps, you're better off making a google calendar, sharing it with him, and actually using the damn thing (adding events as soon as you know about them and asking him to do the same). This might help. Therapy (how can you be so insanely disrespectful to someone you "love dearly"?) would be better, but it sounds like it's too late and you need to do something solid, too.

Show him you mean it. Words will be meaningless at this point.

What is it that keeps you from doing the simple, every-day tasks the we all must do to function in the world? What is the payoff for not doing these things? Once you figure that out, you can alter your behaviors and get that payoff from something positive. You must know that you come off sounding a bit entitled, "I'm so flakey and so special that the world will catch me as I fall, la-de-da! I don't like to plan, it's an EVIL word! *giggle* So I just go around doing what I do and fuck anyone who gets hurt by it, or has to pick up my slack." If your SO weren't in the picture what would your life be like? Would you be sitting in the dark because you didn't pay the power bill? Would your dog be dead because you didn't take him to the vet? Would you be unemployed because you failed to communicate and plan at your day job? Oh wait...you don't have this problem at your day job, do you? It's only with the person you claim to love that you mess up, that you make him responsible for all the stuff you're too-special, creative, lovely, to do. I'm not trying to slam you, I'm trying to get you to see how maddening this must be for him. I'm sure he's special, creative and lovely too, but you're the one that gets to float through life, and he's the one that has to do all the work so you can do it. When I put it that way, it doesn't sound fair. You may need a time management App, or course or book. You may need reminders. Whatever it is you need, to get you to do your share, get it. Actions speak louder than words. So stop making excuses and start being a person who does PLAN. It's not a four letter word, it's the difference between peace and chaos. So why exactly is it that you choose chaos?

Yeah, I'm also not getting why you felt the need to hide this business trip from him unless there's more going on here than you're telling us. Knowing about something for months and not telling your partner isn't "bad planning", it's "deceit". Do you have trouble saying no in general? Are you afraid of his reaction? Would you rather go on the trip than be at home? What's behind this? I'm a terrible planner. This is not that.

To make him feel I will make a sacrifice on my part that for once puts him first. In a situation like this, "I will" is meaningless. The only way you convince him that you have changed is by actually changing. It's very unlikely that he will trust that change is forthcoming, and there's probably not much you can do to sway him on that. Just accept that you have to rebuild his trust with actions, not words.

Seems like you guys shouldn't be in business together. Might be better to minimize the opportunities for your lack of forethought to hurt him.

I agree with the others that say you should get outside help for this, whether through therapy or some other (probably professional) means. This isn't new. This is a constant pattern of showing your partner that your time and your needs and your spontaneity is more important than him, his time, or his feelings. For years. This is so unfair to him. I know this sounds harsh, but that is fucked up. If your partner were the one writing this AskMe, I would expect just about every answer here to be some version of "break up with your partner, who obviously doesn't care about you, which they have demonstrated clearly over and over again." If you don't make some serious, ACTIVE changes, now, then I don't predict anything good for your relationship (or your business).