It’s definitely best to have someone to talk to (in my case, I had my friend reply my messages in Japanese, though we don’t talk a lot in Japanese), but if you don’t have that, maybe try watching anime and catching YouTube videos or streams? Anime isn’t all realistic speech, but it’s not all non-existent slang either. A lot of it is stuff you can reapply by making it more polite. (Almost all my N3-N2 grammatical knowledge is from watching anime and checking the dictionary, and while I don’t have a ton of conversational practice, I can speak somewhat coherently, and my Japanese teacher in school has confidence that I should be able to pass the N1. I’ve been studying Japanese for about 3.5 years, and I watched anime quite frequently the whole time.) I personally watch rom-com/isekai anime a lot, and VTuber streams.
That aside, the resources @ekg shared aside… shameless plug, but I’m doing a series on grammar on Twitter right now. Here’s a PDF compilation of all the Tweets from Part 1:
I’m currently on Part 2, which is about particles and basic verb forms, which should help with forming sentences. Here’s one of my more recent Tweets:
https://twitter.com/AnAsianPolyglot/status/1496410904321265665?s=20&t=bTaGF53Rov8NG0iDC_Ksxw
My aim is to make grammar more intuitive by breaking it down, including by using the history of words and the analysis Japanese people (not foreign learners) are usually taught. If you’re on Twitter, maybe you’d like to join in? 
Whatever you’re using to learn, you can practise on Twitter, HelloTalk, italki, HiNative… and here on the forums, like in this thread:
You’ll probably have a few people coming along to correct you or make suggestions.
That aside, you can ask questions in the Language Questions and Grammar Questions threads if you want to see if a conversational sentence might be correct.
I hope you’ll see good progress soon!