1) Regret about things done or said before should be the kukkucca cetasika, which is a dvesha (Pali: dosa)-related cetasika. I think, like Akvan, that there is also some measure of moha or delusion involved. Therefore it is not wise to let regret predominate in your thoughts. Instead resolve to do better and be a better version of yourself next time.
By replacing akusala cetasikas with sobhana cetasikas such as samma vaca (right speech), samma kammanta (right action), adosa (non-hatred), karuna (compassion) and mudita (sympathetic joy), you will, by using the conditions of repetition and association, gradually change your gathi for the better.
Cetasikas set forth the conditions to bring about other cetasikas to come about and appear. For example, whenever we focus on cultivating a sobhana cetasika such as samma vaca or samma kammanta, we are invariably also cultivating the 19 universal sobhana cetasikas such as alobha (non-greed), adosa, saddha (faith) and sati (mindfulness) among others. The 19 universal sobhana cetasikas always arise in association with other sobhana cetasikas. This comes about while also suppressing the akusala cetasikas in that instant.
Through the condition of repetition, each javana citta will strengthen the next javana citta in a citta vitthi. Therefore you will be reinforcing good gathi the more you practise and cultivate sobhana cetasikas.
2) I think one does have to be intelligent enough to understand the concepts that one has read. However, intelligence by itself is neither sufficient nor absolutely necessary to understand Buddhadhamma. I agree with Akvan that panna (or pragna) is needed in order to attain the Arahant stage.
Although you may forget the concepts after a while and get confused, please continue to study Buddhadhamma. It is highly useful, maybe even critical, for you to develop the four bases of mental power (satara iddhipada).
Through continued vimansa (investigation/reasoning) of Buddhadhamma, you will gradually get closer to the goal. In the event that you do not attain Nibbana in this very life, you may become a highly intelligent person in future lives to aid you in your cultivation, due to the law of kamma (for you had spent your current life investigating Dhamma).
Hope this helps! :)