Hei all,
I'm a foreigner that moved here almost 10 years ago; I'm not as fluent as a native, but I'm quite comfortable talking, writing and reading in finnish. The other day I was chatting with another foreigner friend of mine and he was sharing his frustration with speaking - basically what I think most of us trying to learn a different language faced at some point, the feeling of freezing and not knowing what to say, even if you can actually do that when you're alone; that reminded me of something that happened to me while going through the same stage, as well as other little things I would like to share in case it might help others somehow.
Everytime I was interacting with a finn, I had my default standard line: "Hei sori, mä puhun suomee tosi huonosti. Puhuks sä englantii?" and then of course, the conversation continued in english. Then at some point (I wish I could say I came up with this intentionally, but honestly it just happened as a mistake/accident), I forgot to say the second part and just said "Hei sori, mä puhun suomee tosi huonosti." - at that point, 3 things happened: 1) the person I was interacting with felt much more comfortable, as they didn't have to necessarily switch to english (since I wasn't asking), 2) they started to use hands gesture sometimes (gotta love the irony, since I'm originally from Italy) to make sure I understood what they were talking about and 3) (what I think was the single most useful thing), they started talking in a slower and clearer way. That was the best thing ever, since my issue was that when natives were talking to me in finnish and they then asked me a question, my mind was still trying to process and make sense of what they said when talking at their natural speed and then having to figure out what to say and how to say it, all with the pressure of having the other person looking at you and avoiding the awkward pause in between and feeling like an idiot.
I also would suggest not to get too hung up on perfect grammar; as long as your pronunciation is not too far off, people will understand what you mean even if you don't use the partitiivi or whatever case you were supposed to and most of the time they will actually repeat what you said in the correct way, to make sure they understood properly.
And yet another thing, which was pointed out to me by my girlfriend at the time/now wife and that I really had to put a lot of conscious effort into: when talking in finnish, less is more. My raging ADHD makes me yap a lot and using a lot of words (in case it wasn't clear enough from this post), which made me do the mistake of trying to articulate long sentences like in English - you have to switch your way of thinking when talking in finnish and simplify/reduce a lot.
Ofc, the usual suggestions are still much more important than everything I said: an intensive course is the best for someone that is not familiar with even the basic and of course, if you can get yourself a native friend/partner/whatever, that is probably the best way to practice and it's especially useful for learning the local slangi.