The form "watcha say?" is the colloquial pronunciation of "what do you say?" (what d'you say) = do you think it's a good idea?
The process of assimilation is a bit complex here; let's see it step by step:
0- YOU is often pronounced YA
1- What do you: T+D=T – whato ya
2- The vowel in DO weakens and disappears (what d'you), so we get: what'ya
3- T+Y=CH (this is one of the most common assimilations in English pronunciation): whatcha.
The same thing may happen in all questions starting with "what do you...". We don't write it like this, but we very often pronounce it like this in conversational English:
- Watcha want? = what do you want?
- Whatcha say? = what do you say?

In colloquial conversation we can also supress the verb to be and get the assimilation T+Y=CH with "what + (to be) + you...":
- Watcha name? = what your name = what's your name?
- Watcha doin'? = What you doing = What are you doing?
Of course, this assimilation also happens when we have what+you in correct grammar too:
- Tell me watcha want = tell me what you want
- whatcha need is love = what you need is love

In the second line of the chorus we have another assimilation with T+Y=CH:
-That you only meant well (pronounced: "thachoo only..."). In this case YOU doesn't change to YA (but it could).

J. R.= The name of the producer of this song.

WATCHA SAY?= What do you say? What do you think about it? What is your answer to my words? (I made a suggestion to you, what do you say to it?)

WHAT DID SHE SAY TO YOUR 'WATCHA SAY'?= What was her reply to your suggestion?
He cheated on her (he had sex with another woman) and then he came back and said "I'm sorry, I want to stay with you, what do you say about it?". And now the song is asking: "What did she answer to your question "watcha say?" ?

[did she say] THAT YOU ONLY MEANT WELL? WELL OF COURSE YOU DID= This is a possible answer from her. MEANT is the past of MEAN. To mean well means to have good intentions. "Well, of course you did" sounds here like "oh yes, you say you had good intentions but I don't believe it" (it's an ironic answer).

[did she say] THAT IT'S ALL FOR THE BEST? OF COURSE IT IS= Exactly the same as the previous answer, she is ironic and in fact she doesn't believe it's all for the best.
We say that it's all for the best when we are talking about a bad thing but we believe something good will come out of it, there will be good consequences in the future. For example, if your friend loses his job you can say "don't worry, it's all for the best, maybe you'll find a better job in the future and you'll be glad you lost this job". And then your friend will probably say "of course", which means "oh, shut up, you didn't lose your job, did you? I bet you wouldn't be so happy about it".


I WAS SO WRONG FOR SO LONG= I was very very wrong for a very very long time. That means that he had been cheating on her for a long time.

TO PLEASE MYSELF= To find pleasure for me.

LUST= A strong sexual desire.

I WAS CAUGHT UP IN HER LUST= My desire for her was very strong and I couldn't control it. If you are caught up in something, you are trapped there and you can't escape.

WHEN I DON'T REALLY WANT NO ONE ELSE= Though in fact I don't want anybody but you (no one else = no other person). This sentence has a double negative, so it's grammatically incorrect, but in colloquial English we can find double negatives sometimes. The correct sentence should be: when I don't really want anyone else / when I really want no one else.

TREAT= The way you treat a person is the way you behave to them, the things you do and say to them, so if you treat them right, you are nice to them, and if you treat them badly, you make them suffer.

I SHOULD OF TREATED YOU BETTER= I should have treated you better.
The verb HAVE is contracted in this sentence, so the correct spelling is: I should've treated you better. SHOULD'VE is pronounced /ʃʊdəv/ , so 'VE sounds here /əv/ , exactly the same as OF. That is why it is common to find OF instead of HAVE in some colloquial texts (but it's incorrect).

MEANT TO LAST FOREVER= (mean-meant-meant) If something is meant to last forever, it is supposed to never end.

LET ME IN= Accept me (let me get into your heart, into your life).

ANOTHER= /ənʌðə*/

CHANCE= Opportunity.

CAUSE= Because.

ROOF= The top of a house.

CAVED IN= If a building caves in, it collapses, it falls in and gets destroyed. Here, "when the roof caved in and the truth came out" means, "when my lies were exposed and you found out the truth".

LIVING SO LARGE= Living like a rich person.

I LET OUR LOVE GO= I was destroying our love.

I DO WITH ONE CHANCE= I have one opportunity.

GOTTA= (have) got to.

CHEATING= If you cheat on your wife/husband/girlfriend/boyfriend you have sex with another person.

CUZ= Cause, because. In colloquial AmE you can write CUZ and in colloquial BrE you can write COZ (because the accent is different).

AIN'T= aren't.
This colloquial form is used as the negative of the verb to be or the verb to have:
- I ain't no student = I'm not a student
- he ain't here = he's not here
- you ain't gotta clue about it = you haven't got a clue about it (you have no idea, you don't know anything about it).