How can I help my teen daughter lose weight

If her diet is mainly whole foods, having cheese on a jacket potato or four weetabix won’t be what’s leading to being overweight. Especially if she’s really doing two hours of exercise a day. Yes, she could be over indexing on fruit but even then if you eating the whole fruit the fibre will fill you up pretty fast.

I would guess she’s eating a lot of sugar and processed food out of sight but eating healthily in front of you.

Also what exercise does she do? Most burns a lot less than people assume.

Get her to look up recommended portion sizes and make sure she sticks to that - 2 weetabix with skimmed milk

most people have milk with their weetabix and that along with the weetabix contains protein

support her - she will feel hungry especially with making changes so have things you can do together - “come for a short walk with me instead “ or “help me with this please “

I think key here is for to understand the calorific value of each think she eats and keep a diary. It adds up. Workout what she needs to consume to lose weight.

Then look at making smart choices.
Meal planning and just be aware of what she is eating.
I recommend the Michael Mosley cook books, lovely recipes for about 500 calories a pop.

See is exercising and studying so she needs the traditional 3 meals a day, but upping the protein and reducing the carbs. As you say 4 Weetabix is too much.
Have a tuck box with her daily snacks in. Once they have gone they have gone.

4 weetabix with milk and sugar could be around 400 calories. A banana can be around 100 calories. If she's a hungry girl and she's eating a lot of carbs and little protein she can easily go above her TDEE by 200 calories or more every day without too much difficulty even if she is active. OP hasn't actually said how overweight she is, she might be short and only a little overweight in which case her TDEE could be 2200 or so per day which for a hungry active teen is easy to exceed. There is no major mystery here - she's eating a bit more than she needs on a regular basis.

Is she on hormonal contraception? It causes weight gain in some people through increased hunger. She could switch to a none hormonal method like the copper coil

I think you really do need to be pointing out the things you mentioned above. Its not nagging, its guidance. A jacket potato with salad is all well and good but not when laden with cheese and a whole tin of beans. She needs to know that. 4 Weetabix for breakfast is alot. 2 with skimmed milk and a banana would be healthier. Alot of fruit could be mixed up a bit with some fruit and some carrot/cucumber with healthy dip. How will she know if you dont tell her? Watching her consume it all and then letting her wonder why the scales are creeping up is a bit silly

4 weetabix is only 270kcal roughly - it sounds like she's probably not eating enough protein. Does she count macros? Even just recording how much she gets in a day could help but obviously you don't want to make her too watchful of kcals.

I'd quietly make low calorie fruit options like watermelon & mango available for snacking & up the protein portion of her meal.

I would imagine she is eating out of sight.

It doesn't sound as if she is unhealthy? I would perhaps see the GP as 4 weetabix isn't going to make you fat if you're doing 2 hours exercise a day

I stopped gaining weight when I was diagnosed with ADHD (mid 20ies)

I am losing it currently, the things I’m listing below are a massive help to me.

I am aware that I am very privileged (medically speaking) and that this won’t be feasible for everyone.

things that come to mind/I would recommend based on my personal experiences:

1. do it together. Keep healthy food at home, cook high protein, low sugar meals etc. My SO supports me by cooking healthy, high protein, high fibre meals.
2. offer her a gym membership or other kinds of exercise she may enjoy (dancing, cycling, running coach etc.)
3. get her an appointment with a psychologist/psychiatrist.
4. get her an appointment with an endocrinologist. Cortisol levels, thyroid etc. Go to a competent professional that will take issues such as subclinical cushings into account, tests for everything and won’t just “assume” or shame your daughter.
5. work with a nutritionist. Mine is luckily covered by my health insurance.

edit: exercise is great. But what really helped me was getting a check-up by a cardiologist and the sport therapist recommending optimal training routines for my heart rate, interval training etc.

What about the water? And is she eating the right things to support the type of exercise she does?

Would she write a meal plan with you?

Increase exercise, how much does she do currently? And does she drink enough water?