I used HelloTalk (app) to improve my grammar in French, since chatting to native speakers means that you get to practice using the grammar in lots of spontaneous situations, and so get corrected as and when you need it! It's more relaxed that dedicated grammar learning, too, and I find that it gets stuff into my head more. Another trick is to watch youtube in Spanish with the subtitles on in Spanish, and then take down examples of grammatical structures and vocab that you don't know. Revision guides can also be helpful - I have the Complete French Grammar by Heminway, and I think they do a Spanish one too.
(Just finished Y13 + hoping to do langs at uni next year)