I know it would be hard to sell your horse on to get another horse...I was in the same position with my mare Stoeka. She is old and small (26 years old, barely 15 hands) and I want to do show jumping. She used to do show jumping, but now she won't take me far anymore...
I was lucky because I got to keep her and get another horse to ride competitively.
If it is possible, try and keep her and see what you can do with her. Since you have had her for long and worked with her so long, it would be difficult to say goodbye.
If you have to sell her, or you have kept your mind to getting a smaller built horse, try and make it possible so you can maybe still see her once in a while. Like sell her to a friend, or to someone nearby to you.
Just a thought. Let us know what you have decided what to do.