I agree with Gabrielle56 about clearing everything out first to see just what you’ve got to play with. It sounds as if the clutter is getting you down a bit.
You then might find that a spruce up, fresh blinds and new glazing might just be the job, though sometimes doing this can be almost as expensive as starting afresh. If it’s cold, then you would either not use it or need extra heating to be able to, whereas a new insulated structure (perhaps with a solid roof and possibly a skylight) might actually save money in the long run.
I suppose you both need to decide just what you want to use it for and move on from there.
We have a brick built extension with solid roof which has become part of the kitchen and part “snug”. It gets used all year round. I do wish it had a skylight in it though, as it has made the dining room quite dark (we didn’t have it built), but on the other hand, the dining room is rather cosy in the winter months and we tend to eat in the kitchen more in the summer, so it’s not a massive problem.
Good luck.