Hi Susie - I have no real advice other than to empathise with you. I think it is just as hard being a parent of a sporty child managing the highs and lows as the child itself.
Other than to state the cliches of sport development being a long burn, and will include dud seasons where they have not achieved their perceived potential, I guess my underlying question would be: does she still enjoy her sports, and if so, then she should keep going, as she does with her football, as not all paths can lead to high level success. And that learning from 'failure' is also a key lesson in life.
The lack of confidence is an interesting one, and holds a lot like yours and mine down (vs lack of ability), as they, for whatever reason, lack that killer instinct/aggression/drive needed to not only instil confidence in themselves, but win over others...especially in contact sports. But this is the difference in those with natural leadership on the pitch/pool/court etc vs those that are the team players...just as essential but not always the ones that shine.
We are working on this one ourselves (my DD is nearly 13) and I see it as a WIP vs a quick fix. She's a great team player but always seen as the supporting role to other stars. I did make her read some elite sport autobiographies this summer to give her an insight to their journeys. Not all were pretty reading, but actually did help her to put things into perspective, including peaks and troughs, body changes, juggling school, commitment etc. I got a thanks for doing this mum for the first time last week when dropping her off at the pool last week. Not to say she doesn't have her appreciative moments in life but penny dropped on the impact it has on all of us supporting her and spending countless weekends reading a book on the side lines 
I am not sure if it helps, we are pretty blunt in our household, and use analogies such as the lead singer in the pop band always takes the limelight, but would be nothing without the other band members. Same with music, not everyone can be the soloist. This occasionally results in a reverse where she gets a spurt on...it's sustaining this that we are challenged with!
Final point re your DD's coach - we have had this feedback a few times. We moved coaches on one of her major sports, and out of sheer determination to overcome his feedback, she is now starting to make some in-roads. Not a natural in this sport, but she has been recognised by her current coach as someone that can still succeed, due to her hard graft and attitude. And that 13/14 is such a critical stage where children drop out due to lack of form, that getting through it can work for some, with support.
So try not to get hung up in the moment only, and see if she can come through her low patch, and try to put everything into perspective, including what she really wants to achieve out her sporting career.
Sorry that was so long!