My advice on top of Lieven’s would be to practice with fretting hand muting strings, don’t focus on chords and chord changes until strumming feels comfortable to you. Well at least when you practice consistent strumming, of course chord changes are important too but you can dedicate 5-10 mins a day just strumming with empty clicks while muting strings with fretting hand 
Slow, slow, slow. Slow it all down, don’t try to play a song, mute the strings and just focus entirely on what you’re doing with your strumming hand. 30bpm, 20bpm, less… however slow you need in order to hold the pick correctly, get it at the right angle and distance, and move it accurately up and down over the strings without making mistakes.
Then once you get good at that, speed it up slightly and start again. Rinse and repeat and if you practice for a few minutes each day, you’ll soon be strumming well at whatever bpm you like.