Maternity cover (uk)

So I have just been sent across information to process my maternity cover - FTE she is being paid £8000 more than me a year. Looking at her CV I can see she is very experienced and don’t question her worth however she is coming in to cover my level of role and she will be working 1 day a week less than me and still takes home considerably more. My boss has justified her pay by saying she will cover my job and added duties in this time.. not sure where she will have the time unless I’m really bad at my job (lol?)

Would you feel put out by this or am I being sensitive? I also am currently underpaid and have asked for a pay rise twice in the last 6 months which was declined. My boss assured me when I returned she would put me through further training and development however now has plans for this cover to stay and move into a more senior role when I return - leaving me back in my job on a wage lower than I now know my business can afford.