Students self-assess which exercises they attempted – how can I make this more homogeneous?
If the problem (being too pessimistic) is not too common, let the student check the exercises not only in the usual way, but also with "?".

Announce at the beginning that if students are totally unsure whether or not the may check an exercise, they may check it with "?". Then, whenever a student checked an exercise with "?", let them present this exercise (with no negative consequence whatsoever) and then say, if the attempt was resonable enough to warrant a checkmark. Announce when explaining "?", that students with "?" will have to present. (This has also the advantage that students see a not-so-perfect solution which is often more helpful and more intuitive then a perfect solution.) However, this only works if exercises do not get too often marked with "?" (as otherwise, this system could be gamed).

Some additional thoughts:


In my experience with similar systems, the "pessimistic" students are often the excellent ones, that is, the ones that pass the course anyway.
A lot of students have exactly the number of checkmarks they need to pass the course (and not more). I do think (but without proof) that if students are just at the border between passing and failing, they will do enough work to pass and get more "optimistic".
Announce in the first lesson as clearly as possible when an exercise can be checked and also, when it cannot be checked. It is often useful to give examples when it can not be checked as students may compare their situation to the example.