feeling down about being a SAHM

First of all, I love being home with my 7 month old. I chose to be home with him & hate to leave him so I couldn't imagine working outside of the home without him. I got pregnant in college. We met in college & by the time I was pregnant he already had his degree. We got married & I pretty much dropped out. I told people I was going to do online classes or finish after the baby was born, but never did. I have no motivation to being a SAHM. I'm drowning in student loans as it is, I'd hate to make more when I'm not even working. Today all of my classmates graduated with their bachelor degrees. And I'm feeling pretty down about it. I'm a 21 year old wife & stay at home mom & for the most part, I'm happy. My husband is wonderful and my child is my everything. But I'm so lonely. I have no mom friends or even married friends because I'm so young. I can't help but envy my husband a bit who is staying in the same track he always dreamed of and achieving all his goals while I'm at home changing diapers. I feel like a loser. I love being a mom more than anything but it's such a sacrifice. I'm embarrassed to admit how I feel because I feel like a bad mom for feeling this way. I do SO much. I'm so tired. From breastfeeding 24/7, cooking, cleaning, running errands, raising a baby. I'm so exhausted but yet I feel like nothing Indo really shows. I truly believe that being a mom is one of the most important jobs in the world but it just doesn't feel like it. I'm just feeling down & needed to vent. How do you find purpose as a SAHM?