Im 21 and so sick of being single..?

im a 22 yr old girl and ive never had a real relationship. ive dated guys before but never for very long and they usually end weirdly. like, he will just stop talking to me all together and thats that. i try to pretend like it doesnt bother me. i act like i can deal with just hooking up with guys and never having emotional attachments. but its really starting to bother me lately. all my close friends have boyfriends and its just a reminder of my aloneness. i have such low self esteem and i shouldn't. its also really hard for me to meet guys. im not very outgoing. i usually only go places with my sister and she is wayy prettier than me and usually gets all the attention. i work at a preschool so no meeting anyone there and i am outta college for the summer. i just am wondering if anyone can give me any kind of advice.. i would really appreciate anything.