The difference between 하는데/하지만 and 했는데/했지만 is the present versus past/perfect tense. In Korean, the past form -ㅆ다 can also represent the sense of completion or experience like the present perfect in English. -지만 and -는데 can both introduce a contradicting fact, but -는데 has a much broader usage. Unlike -지만 which is always dismissive of the clause (i.e. it implies dropping the idea or putting up with it because there's no choice), -는데 can suggest the mentioned situation holds true as the backdrop of the main clause. It thus can make sentences -지만 cannot. For example 1. 비(가) 오는데 왜 우산(을) 안 가지고 가? (O) = Why aren't you taking an umbrella WHEN it's raining? 2. 비(가) 오지만 왜 우산(을) 안 가지고 가? (X) = bad logic - it makes no sense. -는데 in #1 is like "when" in English indicating a background situation for the main clause.

above all, in korean there is not grammer like 'have p.p'. so in this case both past and present are OK. that is, there is no big difference. you can use all three. word '하지만' focuses on difference between former and post content word '하는데' is used when you want to change topic but tell you the truth, many koreans use both not differentiating them anyway if i were you, i can use this : 내가 한국어 공부를 많이 했지만 잘 하지는 못한다