Do doubt and repeated swearing in prayer and fasting, and saying, "If I get dental braces, I will not enter Paradise," prevent one from entering Paradise? Is this from the whisperings of Satan? And what should I do?

The cure for obsessive thoughts (waswas) is to ignore them and not pay attention to them. Any oath uttered under the influence of waswas is not binding. As for oaths sworn by choice, they require an expiation for an oath (kaffarat yameen). A person should do what is better for them. If the oath was sworn by choice, then expiation for an oath is due. If it was under the influence of waswas, then nothing is due.