Is there a word for telling someone all your thoughts? For example, when you've been trying not to talk about something but you end up blurting it all out in one big mess of emotion and it's all over the place and almost desperate. Or just a word that describes someone desperately yelling all their thoughts to no one in particular; they are just getting their thoughts out.
It's definitely "colloquial", but from Cambridge Dictionaries...
unload - to tell someone about your problems, the things that worry you, etc.
(e.g. - I've been unloading my worries on poor Ann here)
The example usage unloading my worries above clearly shows how this particular metaphoric usage came about in the first place, but almost 2000 written instances of unload on you in Google Books should be sufficient to show that the intransitive usage is well established.
How about vent?
vent: to give free play or expression to (an emotion, passion, etc.): to vent rage; to give public utterance to: to vent one's opinions; to relieve by giving expression to something: He vented his disappointment by criticizing his successor.
From the OP's example:
"When you've been trying not to talk about something but you end up blurting it all out in one big mess of emotion and it's all over the place and almost desperate," you are venting your pent-up emotions, confusion, anger, frustration ... or you are simply venting.
It isn't a single word, but consider the idiom "spill ones guts," as shown in Idioms by The Free Dictionary:
to tell all; to confess;
to tell secret or personal information
to tell someone all about yourself, especially your problems
You don't currently give an example sentence, but based on your description here is how it might be used:
After all of Bob's badgering her over her recent hesitance to talk, Alice was ultimately compelled to spill her guts.
Its not a single word, but how about "brain dump"? The most common sense means explaining or writing down everything you know about a subject. It is generally used to refer to knowledge rather than emotion though.
To "bare one's soul" to someone would seem to cover the OP's situation although it's not one word.
He felt a great sense of relief after being asked by his doctor to bare his soul about his innermost worries and anxieties.
To bare one's soul: reveal one's innermost secrets and feelings to someone. (Google online).
Speaking all thoughts, as they occur?
Not a single word, but how about Stream of consciousness, referring to the verbal narrative mode (rather than the actual awareness 'streaming' through your head.)
Another might be: full disclosure. Imagine two people watching a ranting lunatic across the street who yells out every thought as it occurs. They look at each other, and the first one says "Full disclosure?" The second smiles and says "TMI!"
lay bare one's soul
M-W Learner's Dictionary:
1 a : not having a covering
— sometimes used figuratively
He laid bare his soul. = He laid his soul bare. [=he revealed his most private thoughts and feelings]
You could also use the phase verbal diarrhea. It has some strong connotations to both desperation and a "big mess of emotion" like in the question.
NOUN, vulgar slang
The fact or habit of talking too much:
'was it necessary to have the narrator exhibit verbal diarrhea throughout the entire picture?'
- to issue copiously or violently
- to make an effusive display of affection or enthusiasm
Example: I'm tired of hearing her gush about her boyfriend.
M-W.com I removed the inapplicable definition
You are ranting:
speak or shout at length in a wild, impassioned way.
"she was still ranting on about the unfairness of it all"
synonyms: fulminate, go on, hold forth, vociferate, sound off, spout, pontificate, bluster, declaim; shout, yell, bellow; informal: mouth off
"she ranted about the unfairness"
a spell of ranting; a tirade.
"his rants against organized religion"
synonyms: tirade, diatribe, broadside; literaryphilippic
"he went into a rant about them"
If someone has been keeping something to themselves for a long time it can cause them a lot of stress. A friend might then come alongside them, see that they need to divulge their private thoughts, and suggest it might do them good to 'offload'.
'Offload: Relieve oneself of (a problem or worry) by talking to someone else'
Another, closely-related word would be
Unburden oneself: Relieve (someone) of something that is causing them anxiety or distress: 'the need to unburden yourself to someone who will listen'
Open up [to someone]
To start to talk more about yourself and your feelings.
Use in a sentence:
I've never opened up to anyone like I do to you.
Unload, venting, and especially ranting have the connotation that the information is negative and/or the person giving it is exaggerating the impact.
"get this/something off my chest" or "blurt out" give the feeling that the speaker has been holding back on something serious and has finally said it while leaving it ambiguous if the thing is positive or negative.
He said, "I've got to get this off my chest. I've been offered a promotion, but we'll have to move to another city."
At Dinner, he blurted out the news of his promotion.
One of the idiomatic uses of "dump" has a meaning like this. From Dictionary.com:
dump on (someone), Informal.
b) to unload one's problems onto (another person):
You never phone me without dumping on me.
From The Free Dictionary:
dump something on someone
Fig. to pour out one's troubles to someone. She dumped all her grief on her friend, Sally. I wish you wouldn't dump all your problems on me.
The advantage of these idioms for your intended use is that several of the ordinary meanings of "to dump" ("to drop or let fall in a mass", "to unload or empty out [...], as by tilting or overturning", "to transfer or rid oneself of suddenly and irresponsibly", again from Dictionary.com) are suggestive of the act of suddenly and messily blurting out many emotional thoughts.
The disadvantage of the first idiom is that the other idiomatic meaning of "to dump on someone," which I think is more commonly recognized, is to criticize someone, and the disadvantage of the second idiom is that "to dump something on someone" often means to suddenly give someone an undesirable task to perform. In order to avoid those connotations you would have to be careful about context and framing. (On the other hand, if those connotations are consistent with the act of blurting out the things in question, you might not need to try so hard to avoid them.)
Vent is pretty good, but how about divulge?
2 : to make known (as a confidence or secret)
It means exactly what you want, but it has the added advantage of being something like an onomatopoeia, bringing up mental imagery of a dam breaking, and info flooding out.