hm, I had friends at university - it usually happened that at least a few of us had the same/similar breaks and we went to eat something or to grab a cup of coffee/tea or just to chat at the uni premises or in the park or went about town etc.
Sometimes I was on my own too and went to the library (to do homework assignments/read on uni stuff or just stuff for personal interest) or read a book or magazine etc. If went to lunch alone sometimes also read or wrote in a journal etc.
People think girls who write are cute
I pretended to be a mysterious writer girl haha..
In a 2-3 hour break you can also go shopping or such. Or just look around shops/boutiques, try stuff on, browse book shops... (or whatever stores of your interest)
Even if you don't intend to buy anything, you can figure out what looks good on you or what other people wear etc.
Even in primary and secondary school, I always sorta had people I could 'hang out' with, at least a few... You can learn to make that... Just be nice and smile and ask any questions related to school/uni/work activities or immediate surroundings and such...
For group activities/projects at school usually we were sorta 'determined' by the teacher, or people sitting closest got together or friends/acquaintances... Or you could try to find people with same/similar interests...
Are people around you not-so-interesting/unapproachable/'snobby' or you lack common interests/topics to talk about or some good conversation openers?
If the nature of your work makes it difficult to have breaks at the same time as other interesting people, just remember you're paid to work and not to socialize there...? (You can actually get reprimanded for chatting with others at a job, a co-worker wanted me to NOT socialize at a job!!) At most jobs they just don't care either way... You can ask older/more experienced people for advice if you're new/er or encountering new and not-so-well-known work etc. (This depends a bit on what kind of 'atmosphere' is at work, some teams are good, some more individualistic...)
As for 'what would others think if you're always alone' - I think other people are too busy with themselves and don't go around wondering about other people much... You may have tons of friends outside university/campus/work, for all they know... (a roommate didn't socialize at dorm, she had bf and friends at home...) Nobody knows, ya know? You can be this interesting mysterious person they'd like to maybe get to know better...?