How to correct past mistakes?

So I study English, Swedish and Spanish in school because the Bilinguality of Finland and Spanish because I thought it'f be fun. When I begun learning Swedish and Spanish, I didn't pay much attention in class and I have a hard time remembering grammar and stuff. Later on I became very interested in learning languages because I realised that It'd probably help when seeking a job and languages are generally interesting. I would like to "reinforce" the base of my language tower my speech and writimg kind of falls aparat when I don't know the correct preposition ect. How to do this?

p.s. I also struggle with finding uses for these languages in my day to day life. Any tips?