Hello -
45 lbs for a female is not abnormal, just on the small side. But since you are saying she is skinny, I will take your word for it. However, a one-year-old still has some growing left to do. How many ribs can you see? Can you see her spine? What else makes her look skinny?
What's Mixer? Is it kibble? If so, what brand do you feed? What are its ingredients? I am not from the UK, but perhaps you or other UK members can also speak to the quality of this food and perhaps suggest higher quality foods, if appropriate.
Also, I assume you mean by force feeding that you tried controlled feeding - that is what it sounds like. Is that so?
I also wasn't clear on what you meant by "varying her diet" - were you giving her other kibbles (assuming that's what mixer is

)? If you were, how did you introduce them, gradually or cold turkey? If you are talking about other things (i.e. yogurt, eggs), what exactly did you give her that gave her diarrhea?
I know it's a lot of questions, but it helps to understand exactly what is happening. Once you answer these questions I or someone else should be able to suggest something.
Mariah.