I'm not a psychologist but through work I've had to learn and use bits about it and I've got a few friends who studied psychology at uni. It's not a subject for everyone and some of it can be a bit naff and nonsensical, but being able to sometimes get inside someone's head can be useful and gives you an understanding if how and why they're the way they are. With Trump there's been enough wrote and said about him along with how he's been behaving you can get a good feel inside his head. But contradictorily he's both simple and difficult to understand and interpret.
He's a thin skinned, envious, egotistical, narcissistic, ignorant, crass, capricious, vengeful, moronic bully, surrounded by sycophants or people who owe him for their status.
He hates Obama because Obama took the piss out of him at a press club dinner once, so he's tried to undo everything Obama did (the Paris accords, Obamacare, the Iran nuclear deal). Obama got the Nobel Prize, so he has to have one as well so he can't be out done. He hates green energy because he didn't like the look of the offshore wind farms near his Scottish golf course and the Scottish government wouldn't get rid of it. His vanity means he wants Dulles airport and Penn Street Station renamed after him, as neither has happened he's withholding government funds from New York out of spite. His ego means everything he does or has gas to be the "best", "tremendous" or most "beautiful" even if his taste is crass and vulgar. He knows (or thinks he knows) the price of everything but knows the value of nothing, so long as it's shiny and expensive looking it's good to him. He believes that Trump Tower has more floors than it actually has and because his ego meant it couldn't be out done by other buildings in the area. Ex staff like Rex Tillerson and H R McMaster have calling him "a ****ing moron", his professor at Wharton College called him the "dumbest student" he'd ever had. His ignorance means he has no understanding of anything that doesn't interest or enrich him, but he still thinks he's the most intelligent person in the room. If he'd not heard if something and has just found it out, he believes no one else knows about it either until he tells them. His ego means he still sees a 50 year old fairly healthy and fit him when he looks in the mirror and hates being seen as anything other. He's used his wealth and armies of lawyers to bully and intimidate people throughout his life, most New Yorkers hate him (even Republicans) and his business practices of reneging on paying contractors is well known. Plenty of New Yorkers have stories about him and how he's behaved to people over the years, bankrupting smaller businesses by not paying them or tying them up for years through expensive lawyers until they give up. He only views the world through a transactional lens where there are only winners or losers and he's only interested in "winning". He'd rather scam you to earn $50 than work with you to earn $200 because if he scams you he sees himself as smarter than you. He has what used to be called "peasant cunning" or "low cunning", he'll try to sneak and cheat to get ahead and it's your fault if he does cheat you.
Being narcissistic and vain he's open to flattery, playing his ego will get you somewhere with him, such as the second state visit here (technically the 47th US president was invited to a state visit and he just happened to be also the 45th president), you can manipulate him. BUT he's thin skinned enough that if he doesn't get what he wants he'll turn on you. He'll lash out when he can't get his own way and try to bully and intimidate you. And if he does get what he wants out of you or you're no more use to him, he'll dump you like a hot brick. There's no long term plan or forward thinking, just what he wants now and what suits him now. He's only intimidated or scared of people with more money or power than him and admires wealth, power and status.
All of that makes him easy to understand but his thin skin, spitefulness and mood swings mean he's also massively difficult to understand. He'll change positions or what he's thinking depending on how he feels. You're never sure exactly what he's going to do and honestly I don't think he does either. He insulted our armed forces one day, gets a phone call from the King and changes his mind and praises them. He's went from claiming he's won in Iran, that he didn't need any help to demanding help and insulting other countries for not helping him in an illegal war and all in a matter of days if not hours. You can praise him and support him all you want but he's just as likely to hit your country with tariffs or threaten to invade you the next minute depending on his whim. He blusters and threatens because he's got away with it most of his life in his business without any blow back so he still thinks it can work now but he'll fold under real pressure and scapegoat someone to save his neck (Pam Bondi was thrown under the bus and Hegseth is being positioned to take the fall for Iran).
