I´m currently in Granada, in the middle of my semester abroad in Spain. I spent the first semester in Bordeaux, France.
I can say that a lot depends on you and how much effort you put in to meeting Spanish people and speaking Spanish and staying away from English speaking people.
Living with Spanish people is important - don´t become like other Brits here who purposely sought out an English speaking housemate and spend all their time in Paddy´s bar.
The problem with Granada (and I suspect with Seville as well) it that there are a lot, and I mean a lot of Erasmus students here and no doubt you will have classes with other English speaking people.
You could take the easy way out and just fall into hanging around with "the group" or take the hard way out, but more rewarding.
There are tons of students in Granada and the good thing about that is that there are a lot of things to do and for free too. Tapas are so cheap (in Sevilla too) and going out for coffie is quite popular too. Take every opportunity you are presented with to go out with Spanish people (and even other non English speaking Erasmus students as well). Speak as much as you can with whoever you can and don´t worry if you are making mistakes or not - you will fix this naturally with time.
I honestly don´t think that 10 months in a country is enough to really grasp a language. I personally think that you need minimum 2 years to be able to speak really fluently. But 10 months for those who work on their language is 10 months nonetheless and you will come out of it a changed person person linguistically - if anything your confidence level will shoot right up.
As for the accent and going to the south, yes, it is true, it is more difficult here. The endings of words are not pronounced and that means that the Andalucian speaker speaks faster than the northerners. However, that has advantages. While I still struggle with to understand the people here, I now understand the northerners so much better. It is such a treat to converse with someone from the north now, I can´t believe how well I understand them! You get used to working extra hard to understanding the people down here that when you speak to a northerner you suddenly understand without much effort.
I have only been here just over 2 months and my vocab has improved so much I can´t believe it. I´ll tell you what I do to improve it. I do study vocab lists both on the internet and in books. I also read all the student newspapers that I pick up around the university - there is a Spanish-wide monthy student newspaper and one for Andalucia as well. I pick up anything that I can find really including old metro newspapers - it doesn´t matter that they´re old and Metro is not too difficult to read and the articles are short. Then I look up words I don´t know. In the beginning you are looking up loads of words but then as time goes by and you start to see the same words over and over again they start to stick and you understand better. I even study the dictionary a bit before I fall asleep at night. Also, hanging around the youth section at your local library helps as well.
I do study grammar a bit as well. But I mostly do that in the intensive Spanish course that the uni here offers to Erasmus students. If there is one in Sevilla do it because I have learned a lot in it and the atmosphere is non-threatening - everybody else is in the same boat as you and even better, everyone speaks Spanish together because everybody comes from everywhere.
And then I put into practice what I have studied when I speak to people. I have found that I do use the vocab and grammar I learned when speaking to people. Don´t be afraid of speaking to other non-English speaking Erasmus students, especially ones that speak better than you as while you wouldn´t learn much grammar from each other, you do teach each other vocab.
The uni organises language exchanges here every week, sometimes twice a week. A bunch of people show up and you circulate the room talking to people in English and Spanish and I have actually made a good Spanish friend one day. If Sevilla has one of these, go to it. Don´t be afraid, there will be other people like you there as well and everybody wants to talk and get to know you. You never know right? And here in Granda there are free films everyday - go to them as well as it will improve your listening skills.
That is what I do to learn Spanish. I think the ´´problem´´ with your supposed lack of speaking skills is lack of confidence. And lack of confidence from lack of practice. I think, from what you have said, that you have a solid base and you just have to put it into practice. Practice, practice, practice and speak, speak, speak.