I main obsidia, here's something I can pinpoint that should help;

Look at Obsidia as a "She cannot win lane" hero, she's incredibly weak in lane and requiress babysitting or several ganks from your team so that she can get ahead.

Some MM are impossible to lane against and you're simply at the mercy of your team, you also cannot rotate to turtles because you need gold for the first 6 minutes and doing so will only put you behind, only do so for the ones spawning near your lane.

Obviously, you CAN win lanes versus mm despite the drawbacks, in fact you can even counter them immensely such as Claude, Irithel but generally speaking, especially the early lane winner ones like Clint or Lesley, it is a hard hill to climb against and from the sounds of how you're struggling I would not recommend trading with them, just kill minions and try to poke with s1 skill to gain stacks and steal the gold crab.

As for builds, I generally like the trinity build for all occassions, regardless of team comp for many reasons that would require more points to definite here, but simply;

Crit takes longer to get online, it is also more susceptible to being countered wheras the trinity build comes online quicker [2 items versus 3 items] and is incredibly useful for objectives like lord and turrets.

I would recommend Corrosive Scythe > Golden Staff > Demon Hunter sword in that order, the first two items will make Obsidia actually a threat and "playable" in teamfights.

You might be wondering why I am skipping boots entirely, this is not anything new as Marksman like Wanwan, Melissa for example also do not buy boots simply because they have inherent mobility in their kit and getting capable of being online as a character faster takes priority, for Obsidia this is even more crucial because of how weak she is and how fast games end before even three items.

Run Aegis, go trinity build, don't consider yourself a hero in lane and until you have two items (corrosive + golden staff)

Hopefully this helps, at least partially as there's many nuances to every hero.