Jellei, in this sentence (as in many others) I couldn't hear the difference between myślę and myśle. I've beein noticing that for a while, now that I am in Poland: people pronounce the ę and the ą with a defined, round and rich sound, which is completely lost here in duolingo. Is it because of the lack of definition in mobile phones or is it an option of yours? If so, it is a pity, because this sound is so special! Ą is similar to portuguese ão.
The majority of speakers reduces word-final [-ę] to a plain [e]. Some speakers retain a slight nasality. Pronouncing it as a full nasal diphtong is considered hypercorrect, so you shouldn't do it, unless you're playing in a theatre.
Word final [-ą] should be pronounced as a full nasal vowel, but some speakers reduce it anyway to either [oł] or [om].
In this sentence there shouldn't be any ambiguity, since "myśle" doesn't exist. The correct third person form is "myśli".