"I love my boyfriend."
It is a well known point that "with-Duo-only" you cannot learn everything. It is good for pattern learning, but not so good to stamp vocabulary in your memory. Not to mention grammar... The positive thing is that you must be searching by yourself, which is not bad. Reading all these comments is also good: I am not alone!!! LOL
ya not really helpful to make you remember sentences and not really teach you how to form them. Also they introduce kanji but they don't give you the hiragana and often the definitions vary from the ones in the dictionary. But it's the best out of the free stuff so we stick around. You don't really have to go so hard for an app (unless they're paying you lol).
The boyfriend is the object of the sentence (the one being loved), not the subject. Using は would mean that the boyfriend is the subject of the sentence. "(My/the) boyfriend loves" with no object (although context would probably indicate an object of his affection). Ommitting the subject usually implies that it is the speaker.
Wa actually indicates the topic, not the subject. So, if it was "Kareshi wa" it would just be saying "as for my boyfriend..." and could be either "he loves" or "I love him" depending on the rest of the sentence ("watashi wo" and "watashi ga" respectively, as a quick example.)
In every Japanese class I was told to never use "愛をして" because it is too strong, because as foreigners it is a bit hard to understand Japanese way of thinking (like parents usually tell a child to work hard instead of telling them how much they love them and so on) and here we are... I can't recall how often they told us at university to please not use it. A Japanese teacher said it made her so uncomfortable when she heard students saying it... we should please stick to 好き or 大好き. But oh well...
Even with romantic love, they tend to use 好き or 大好き... 愛 is a very forceful/strong love, someone else said it's something you'd hear when someone's on a deathbed, or maybe in a story where they might never see someone again and it's incredibly strong... I learned in class that they confess their love by saying, in literal translation, "I like your things" to be indirect and not too forward about it. If even in confessing they don't tend to say 好き or 大好き then this feels beyond even that. Some teachers described it as "PASSIONATELY love" (capitalization to bring across the amount of force behind it)
Technically 愛し means love and 好き means like, while 大好 means like a lot. But in practice 大好 is used to mean love and 愛 is rarely used. A side note, when my Japanese boyfriend told me in English that he loved me, I asked him to say it in Japanese, wondering if he would use 好き or 大好き. When he used 愛し I knew it was really serious to him!
It's two ways of referring to yourself,
自分 means "oneself (myself, hisself, herself, yourself)"
While 私 always refers to the speaker, 自分 refers to the person who is the subject of the sentence
Either phrasing can be used, though in the first instance 「implied 私は」私の彼氏, you would have 私 referring to the speaker as the subject, and then once again referring to the speaker as the one with the boyfriend, so it sounds a bit redundant (though omitting the subject here removes some of that redundancy).
Whereas 私は自分の彼氏 uses 私 to refer to the topic/subject and then 自分 "oneself/one's own" to show that the subject is the owner of the boyfriend without repeating who the subject is.
Replacing the pronoun with a name to show an easier comparison in English: it would be like saying "[私は] Maria loves [私の] Maria's boyfriend" vs "[私] Maria loves [自分の] her own boyfriend"
This stackexchange goes a bit more into detail: https://japanese.stackexchange.com/questions/4396/why-is-%E8%87%AA%E5%88%86-used-instead-of-%E7%A7%81