First heartbreak

Today, I was wondering how my life will be like (suggestion, I don't think omitting "like" is wrong, but it sounds clearer this way in my opinion) when I get old (suggestion: "when I am old"; "get old" is fine, but it sounds a bit clunky) and [comma] when I remembered something bittersweet. (Using two different tenses when saying "I verb and verb" sounds weird to me, so I made it two independent clauses)

When I was sixteen [comma] I made a promise with whom who (it's tricky to tell here, but I think "who" is the subject of the phrase "x used to be" and that the object of "with x" is an omitted "someone" like "with someone who used to be"; hope this makes sense) used to be my best friend: we would get married, if we are were (the rare English subjunctive! I think some people don't use it anymore, but apparently my area does since it sounds better to me like this) single at (suggestion: "by" instead of "at"; not sure why I think it sounds better) the age of 40. We got the idea from a Friends [italicized "Friends," according to convention, since it's a TV show] episode [comma] and we thought it was perfect. He was never satisfied with any girlfriend or relationship but didn't want to die as (doesn't quite sound wrong, but you don't need it) a lonely man; I was a ("a" sounds wrong, "forever alone" is an adjectival phrase) forever alone and didn't want my dead body to be found after it starts to stink by a stranger. Plus: [comma] we both were born in magic believer towns that believe in magic. (I think this is what you mean?)

Sadly, two years afterwards, we stopped being friends. It happened so fast, [technically you need a semicolon or colon here] one day we were brother and sister [comma] and next day we hated each others. That was my first and biggest not-family-related heartbreak. My mom thought I was suffering because I was in love, but I wasn't [comma, though you might not need one here] and neither was he. Losing your best friend hurts. 

By today we were being friends for more than ten years (I'm not exactly sure what you mean here? Maybe, "By today we would be friends for more than ten years [if we never grew apart]" That's my best guess) [comma] and although we have said sorry already, we are strangers now [comma] but we are good with it. Accepting a lost loss is part of growing up.

(I couldn't figure out if I could do colors, since I'm on an iPad right now; I hope this is readable)