Thank you
AS maths isn't so bad as it's fairly basic differentiation (from what I can remember, not sure if you now do integration as well), but it is definitely a big step up from GCSE. I would recommend that you study the textbook yourself rather than just learning in class as you never really cover all the content unless the teacher is really, really well-organised.
This way, you have a lot more time to really understand the reasoning i.e. what differentiation actually is, the relationships, the main x, y equations to find the values given gradients or lines. I would really stress the importance of doing (all) the questions immediately after learning the content each time, and kind of not giving up until you absolutely understand why you're doing what you're doing.
Once you have worked your way through all of the textbook content and questions at your own pace, I would only then recommend you start past papers. It's really important to get a good 'foundation' so to speak, before you apply it otherwise you just end up really confused because of gaps in your knowledge/logic. I would stick to the time limits at first, just to assess your level and pace, then mark your answers, review it and work over the things you got wrong. Once you've done all the past papers, I would then suggest the soloman papers. From what I remember, I don't think they were much harder, they just ask the question in a different way which tests your ability to understand what they're asking/applying your maths.
As you go along the papers, you'll easily see where you're not doing as well, and you can also spot key equations you need to memorise. It's a really good idea to do a few summary sheets of the key stuff to remember. I think for AS maths I did about 3 double sides of just equations and relationships that were useful so I could refer to them when I was doing a question or was stuck. It just makes it easier to work out where you're going wrong if it's all on the same page.
I hope this helps, but honestly I think you'll be absolutely fine with 3 months, just don't be complacent with your learning and really force yourself to carry on even if it gets really tedious, and NEVER settle for 'my answer is about the same'. Don't stop until you are getting full marks on each paper marking harshly, this will get you that high 90s. I know it sounds like a lot of work, but just tick things off, one page at a time and don't think too much about how much you have left to do and it'll be done in no time! Maths is one of those things that exactly reflects how much effort you've put into it, it will click eventually
I got 100 in C1, 96 in C2 and 99 in S1 doing exactly this after getting an E the first time round (I had a lot of personal issues and just an awful teacher that left me really uninspired).
You can do it.