Van Assche, ranked 47th in the world, pushed Shelton hard and took the first set in Saturday's semi-final, before eventually losing 5-7, 6-3, 6-4 to the American in front of home fans.
"It is true that this week is very important for me because I have a lot of high-level matches under my belt in a row. I played Shelton, who is number four in the world," the 22-year-old told reporters after his defeat.
"That showed everything that I could do well, and where I have some room for progress. This will help me to have more self-confidence; I know now that I can reach a semi-final.
"I know that I can go toe-to-toe with a player like Shelton today. So there are a lot of positive things to get from this, and I will try to work more to be even better next year."
Van Assche's run in Paris means he will rise to a career-high ranking of 34th in the world when the latest rankings are published on Monday.
Asked if he was aiming to be among the seeded players at next year's Australian Open in January, he said: "I don't think I'm going to think along those lines.
"Of course it helps being a seeded player. I will be 34 in the ranking.
"It's a very good ranking. It's very positive. But above all, I will try to remain focused on my game, on learning new skills, on progressing, and then the ranking will follow."
Shelton will face seven-time Grand Slam champion Carlos Alcaraz of Spain in the Paris Masters final later on Sunday.