For some reason or another, almost all Japanese translate yaseru (痩せる) as "lose my weight". This is not actually correct. I suspect that most Japanese were taught in junior high school that michi o mayou (道を迷う) was "lose my way" in English. (Actually, that's not the best way to say that either, but I'll discuss that later.) I think what is happening is that Japanese, having "lose my way" drilled into their heads in English class, just at "t" to the phrase and end up with "lose my weight".
So, how should you say yaseru in English? Simple: lose weight.
▸ 10キロやせる
lose ten kilos.
Since I started running, I lost almost ten kilograms.
▸ 彼女は病気以来ずいぶんとやせた
She has lost a lot of weight since her sickness.
Or,
She got a lot thinner after her sickness.
I lost a lot of weight--almost 10 kilos--since I started running.
▸ 私は甘いものは食べません. やせようと｟減量しようと｠思っていますから
I don't eat sweets―I'm trying to lose weight.
I don't eat sweets―I'm trying to slim down.
I don't eat sweets―I'm trying to reduce my weight. (Here "my" is okay.)
As for futoru (太る), we say "gain weight" or "put on weight".
▸ 3キロ太る
gain [put on] three kilos.
I gained five kilos when I stopped smoking.
I put on five kilos when I stopped smoking.
▸ 彼は年々太った
He got fatter every year.
He got heavier every year.
▸ 彼女は少し太ったね
She has gained a bit of weight.
She has put on a bit of weight.
▸ 彼は太りすぎているのでダイエット中です
He's on a diet because he is a little òverwéight [a little too heavy, ｟話｠ a bit on the heavy side, (恰幅（かっぷく）がいい) rather hèavy-sét]. (!a little, a bit, rather などを添えて表現をやわらげることが多い)
▸ ウエストが太ってきた
My waist(line) is expanding.
As for michi o mayou (道を迷う), a better way to say this in English is "get lost".
The reason I'm late is that I got lost.
▸ 彼らは森の中で道に迷った
They got lost [were lost, lost their way] in the wood(s).
▸ 子犬が私たちの家に迷い込んできた
A puppy strayed into our house.
▸ 「道に迷っておられるようですが, どうなさいましたの」
“You look lost. Can I help you?”
「ええ, どのバスに乗ったらいいのかちょっと分からなくなってしまって」
“Yes, please. I've had a little mix-up (in) finding the right bus.”
