To explore some of the resources on that list:
On mr barton maths.com you will find Craig Barton’s notes for students age 11 to 16 which older students might also find useful for revision. These notes are colourful and clear with carefully worked examples and are popular with students.
An extensive collection of very clear notes and other resources are available from the mathcentre which was developed by a group from the Universities of Loughborough, Leeds and Coventry, the Maths Stats and OR Network and the Educational Broadcasting Services Trust in 2003. There are many very clearly worked examples, also exercises (with answers).
A second mention for Coventry University with Just the Maths by Tony Hobson, another extensive collection of notes which I have often used used with older students. These notes are very useful for students in their last years at school and at university. There are numerous worked examples and also exercises to try. Looking for notes on second order differential equations recently it was the first site I tried and I wasn’t disappointed!
If you are trying any exercises you can obviously check the answers given in the notes; remember that you can also check your answers with WolframAlpha.
On the subject of WolframAlpha, there are several slideshows on the WolframAlpha page on this blog to help you get used to the syntax and a new one has just been added on differential equations.