Sidestep:

 

There's no doubt Ukraine's army is outnumbered and outgunned - particularly with heavy weapons - by Russia's. 

 

But Ukraine's also been at war for eight years. They have tens of thousands of people with good training and military experience.
The United States has given them $3 billion in military aid since 2015 - and the type of equipment they're sending to Ukraine is already used there, is easy to learn, and easy to teach.

 

Remember the success - in the 1980's - of Afghan mujahideen against Soviet aircraft with CIA provided, 'Stinger' shoulder-launched surface-to-air missiles.
The Americans definitely didn't forget. The US, and other countries including Lithuania, Netherlands and Germany have supplied thousands of modernised Stingers to Ukraine.

 

And - the US has provided hundreds of man-portable, shoulder-carried, fire-and-forget Javelin anti-tank launchers and nearly a thousand (@ $100K each) missiles. 
They're a heavier, longer distance missile system than stingers, use a modern automatic infrared guidance system and are "fire and forget".

 

An inexperienced operator can use one after ~12 hours training.

 

There was a push by the US after Russia’s military buildup on the border last year to increase Javelin training and deployment, and planeloads of them were still being unloaded in February.

Enough of them have been delivered now that the Ukranians are firing them at nearly everything - helicopters, trucks, even groups of Russian soldiers. 

And the British have supplied thousands of close-range Main Battle Tank and Light Anti-tank Weapons (NLAWs) which Ukranian Volunteer militias have in positions they've dug surrounding the main roads.

 

Add to that good real-time intel and comms from the US, and you can see why the Russians are losing so much stuff.
There's a good verified list of it here.

 

Javelin are anti tank weapons, stingers are anti air weapons.

The two are not interchangeable in role.