Although I don't have a degree in math, mathematics is one of my favorite hobbies. We had 4 math courses on our faculty of civil engineering (which consisted of a rough "section" through basic single and multi-variable calculus, linear algebra, and probability, along with some mathematical physics - all laid out in a pretty much non-rigorous manner, mostly without proofs etc.), and I took 2 linear algebra courses on the Mathematical department of our Faculty of natural sciences - sadly, I didn't have time for more, although I'm sure I would go and study math for real if I had the time and the money.
So, the only option is self-study, which I've been practicing for a long while, but it's a bigger challenge since you are forced to think your way through more intensively, and explore and try out a considerable number of textbooks and lecture notes (most found on-line), all written in their own style, and every one of them not necessary suitable for every one of us and for every level of "pre-knowledge".
Since I took linear algebra, I believe I have grasped some basic concepts related to this fundamental topic. On the other hand, I had to go through the basics of calculus on my own, and, although it may only be my impression, I find calculus a bit more difficult in general.
The last 2 months I am going through a set of lecture notes about metric spaces and topology - one found at the University of Dublin, and the other two found on the department of math of my university. I also downloaded problems to solve, since there is no sense in going through theory without solving problems. I find the subject interesting and challenging.
Also, I intend to go through some functional analysis.
To sum everything up, self-learning mathematics requires a lot of time and dedication, but if you really enjoy it, I believe it's worth the effort.