I learnt to ski in my 40s, and I'm not particularly good at it now - I can get down a red, but it takes me ages as I do so many turns!
Anyway, my top tips - I echo Suny - don't try to ski with expert skiers. It's soul destroying. You feel rubbish, and scared, and it is no fun.
Take a break - it's a leisure activity, or a holiday. So if you're knackered, it's OK to stop for a hot chocolate.
Try to find small and quiet ski resorts. - This really helps.
Realise that it's a marathon, not a sprint. Yes, some people will pick it up easily and have no fear, but it's taken me three years to progress from the blues.
Get tuition - I quite like groups. But it really depends on who you are put with. I had a lovely group the last time we went. And it was really relaxed so I only did half a week with the group, then pootled about by myself on the blues and easy reds. Otherwise, private lessons are great.
Be OK with not liking it - I like skiing, and I love skiing holiday i.e. being with the family. And my DH and the kids adore it. I'm kind of happy to do it, but I don't have the passion for it that other have. And that's OK. Maybe because I'm not great at it.
Do what you like - if you like doing blues and greens, just do them. If you like going through the fun park, do that.
If your legs go like jelly, or you hit the wall, just stop!