I'm in the same boat. I was a mouth-breather growing up, and throughout most of the day, I had my mouth open. It was very difficult for me to keep my jaws together when I first started mewing, but it got much easier over time. There are some users here who are skeptical of keeping teeth in contact, but in my experience, it is what feels most like "what my oral posture should naturally be". I would caution against clenching, however. The trick is to keep the teeth together NOT with force, but through a balanced resting position, with a good tongue suction hold, and lip seal (at least, that's what works in my case). It may be the case that you will need to use some light force initially, to build up your muscles, but ideally, keeping your jaw closed should not require force - so, aim towards that. And again, avoid clenching.

Good luck.

@elwynn

Exactly. For me, I am unable to keep them together without clenching.

@elwynn when you say "lip seal" do you also mean "lip suction" as in you're gently sucking your lips against your teeth?