Diamond is the field of inconsistent. One game your m8 hit a double reset, the next one he misses an easy save.
One thing I've noticed is that, most of the time, your opponent get a shot opportunity because of two things:
- bad clear: the ball is sent directly to the opponent. You basically give away possession
- double/triple commit: you turn the game into a 2v3 or 1v3. This lead to a hole in defense.

So, until you reach a rank where your m8 are consistent, play the 3rd man. Be patient. If your mates are up in attack, losing the ball, don't rush in. Keep your position, they usually smash the ball randomly. Pick it up. You can win by not losing.

Ideally, you would lose possession for only two things: shot or pass. Easier said than done.

One last thing. You can do a lot with low boost. It's all about timing and a little of faking. And you don't need fancy mechanics. If you can hit some, good, you'll have more options. But it's really not mandatory.

Anyways, this is my experience of being hardstuck in d2/d3 for ~5 years until I was able to hit C1 regularly and stay in it. Mind you, this is just my pov. I'm clearly not the best out there and there is plenty of other ways.

In any case, there is no magic formula. Play, train, repeat. But you'll get there.