The key to a good translation is getting not just the literal meaning, but the sense of the words across. So I'm going to skip over fairly unambiguous words like "this" and "heart", as well as the grammar, and just focus on the more interesting words.
Senia doesn't feel quite right. To me, senex describes a person, not an object.
"Old" is a word which can have a lot of different connotations, and Latin doesn't have a single word covering all of them. In these sorts of situations I turn to Döderlein's Handbook of Latin Synonymes, which is available from the Internet Archive. It explains:
ANTIQUUS; PRISCUS; VETUS; VETUSTUS; VETERNUS; PRISTINUS. 1. Antiquum and priscum denote the age that formerly existed, and is now no more, in opp. to novum, like παλαιός; vetus and vetustum (from ἔτος), what has existed for a long time, and has no longer any share in the disadvantages or advantages of youth, in opp. to recens, like γέρων, γεραιός, γερούσιος. Hence antiquus homo is a man who existed in ancient times; vetus, an old man. Antiqui scriptores means the classics, inasmuch as the age in which they flourished has long been past; veteres, inasmuch as they have lived and influenced manhood for 2000 years. Cic. Verr. i. 21. Vereor ne hæc nimis antiqua et jam obsoleta videantur: compare with Orat. i. 37. Ut illi vetus atque usitata exceptio daretur. 2. Vetus refers only to length of time, and denotes age, sometimes as a subject of praise, sometimes as a reproach; vetustus refers to the superiority of age, inasmuch as that which is of long standing is at the same time stronger, more worthy of honor, more approved of, than that which is new, in opp. to novicius; lastly, veternus refers to the disadvantages of age, inasmuch as, after many years’ use, a thing becomes worn out, or, through long existence, weak and spiritless. Moreover, veternus, in the writers of the golden age, is only admitted as a substantive, veternum, as lethargy; vetus regularly supplies its place, and denotes more frequently the weakness than the strength of age. Tac. Ann. xi. 14 and 15. Veterrimis Græcorum, and vetustissima Italiæ disciplina. 3. Antiquus denotes age only in relation to time, as a former age in opp. to the present; priscus (from πάρος), as a solemn word, with the qualifying accessory notion of a former age worthy of honor, and a sacred primitive age, like ἀρχαῖος, in opp. to the fashion of the day. 4. Antiquus and priscus denote a time long past; pristinus, generally, denotes only a time that is past, like πρότερος. (iv. 83.)
That is quite a mouthful! But I'm guessing you want "old" in the sense of "used up, worn out". So according to Döderlein, you should go with veternus or vetus here.
A crepundia is a rattle, either the thing given to an infant or a musical instrument like a maraca. Metaphorically, it refers to early childhood.
Latin doesn't really have a great word for "toys". Not knowing any further context, I'd actually suggest a word for "tools" might be more appropriate here; the metaphor of tools being used until they wore out would be easy to draw.
Plasticus doesn't usually mean "plastic" the way we use it today: after all, the Romans didn't have synthetic polymers. It meant "having to do with sculpture", from plastēs "shaper", from the Greek verb for "shape". Modern "plastic" got its name because it could be easily molded into shapes.
It doesn't sound like the narrator's heart is easily sculpted: if I understand right, "plastic" here implies "made of cheap materials intended for short-term use". The closest ancient material I can think of is clay, fragile and disposable.
Tearing me apart
Discrepō is a very polite and peaceful word compared with the violent English. It's much closer to "disagree" than "tear apart". I would go with something like dīscindō, dīruō, rēscindō...Latin has no shortage of words for this. And sēparātim to me sounds like each of the loners and fools is attacking you individually rather than as a group; I don't think you'll need a separate adverb for "apart" if you choose the right verb.
Molestia is again a very mild word. It sounds like these people are going to annoy you by talking loudly in the next room when you're trying to concentrate. It sounds like you want a more severe difficulty, onus is a good word.
("Here comes trouble" is also an idiom in English so it sounds odd translated literally. But adapting that is a stylistic decision, and I don't know a good equivalent, so I'll leave that for others.)
I would use a metaphor here. When you invite someone to your house, and that guest brings others with him (intentionally or not), the uninvited followers are called umbrae "shadows" in Latin. An umbra can also be a ghost or a portent, and in that context onus above can also mean something terrible that's fated to happen to you because of a prophecy.