When another person’s idea prevails on your, and is chosen to be executed, there are three important things you can do.
Focus on the positive aspects and on the potential of the idea. No idea is completely good or bad, so the idea that was chosen does most likely have positive aspects. Understand them, ask the owner to explain them, and build the narrative of what is happening around them. “This is idea has the potential to achieve this” or “I like this idea, because if well executed could deliver that” is a much better option than “I was not the one proposing it and I think it is wrong”.
Be involved in the execution and deliver your best work. This is not about you, so leave your ego and all your regrets behind and be ready to help the best you can. People will remember your contribution, and even if they won’t, you will feel much better for having done something active and practical about it.
Keep your mind ready for the next round. Ideas are always needed, so there’s no need to tank if your idea is not chosen this time (or the next, or the one after the next). Try to perfect the ones you had if they are still on the table, or come up with new ones for a different situation. Make sure to understand if your presentation was somehow flawed, if you could have done anything better, write down ideas as they come and try to elaborate on them, and eventually your brain will be prepared for the next chance.