This is a big factor in losing games. I see a ton of people in stream and my opponents upgrading EVERYTHING they have; you should not be doing this.

I know it can kind of be an OCD thing, but you'll have to do something to ignore it, because when you level up things that are doing zero damage per round (read: Crawlers into Vulcan, Anti-Missile Mustangs, or even Sledgehammers against high ST damage units, etc.) all you are doing is feeding your opponent more XP. If you often lose games due to your opponent snowballing levels on you, this might be why. Unless the HP break point is super important to distract an enemy long enough for something else to do damage (which is very rare), there is no reason to upgrade.

Learn to ignore the bright yellow arrow, because spending cash to help your opponent is just a losing strategy.

On the flip side, if you place a single set of Crawlers down on each flank and he sets Arclights down to handle them - and you aren't planning on committing further to the flank, just sell them. Them being there delays the arclight by like 5 seconds max and just feed xp to it.

Many high level players will do a single crawler flank to a vulnerable opponent and then instantly sell them the next round (or next two round if they flank both sides). Another strategy is to put something dangerous on the flank that the opponent kind of has to commit to, then once he has committed supply to answer the flank, they sell it off - bonus points if it's something that doesn't go well into the rest of your army (for instance a Rhino on the flank when you have mostly chaff on your side of the board, he'll have to get balls to answer it and then they're doing nothing once that Rhino is sold.

Same goes for things like arclights after you place vulcans down - often you can still keep them if they're actually doing something, but if all they're doing is a walking simulator as they try to reach the battle while things are dying all around them, just sell them, get that cash back for an important upgrade or even placing them back, but in a better spot.