Is it normal to have possessive and jealous habits

Id like to start this with a statement: I am consciously aware of my jealous and possessive habits, I have acknowledged that I do have a problem. I have brought this to my girlfriends attention, and we are currently working on getting my head out of the gutter.

I am 21 years old. My girlfriend is 19. She and I have been dating for over a year now, and we have had our share of ups and downs. It doesn't help that I deal with chronic depression, and at my low times, I have low self esteem
Issues.

For about 6 months now, I've been in a slump.. I haven't wanted to go out much.. I haven't wanted to see my friends, I never want to go out with my girlfriend, she and I have spent what seems like our entire relationship restricted to the four walls that create my bedroom.

She has had to move 35 minutes out of town. She is in highschool, and doesn't drive, so she unfortunately does not get to see her friends from my town during the week. This means that she has 2 days to spend time with myself, and her friends. she is the type of girl that likes to smoke marijuana every day. I do not smoke at all, and I usually have a problem if she is high around me. I have come to accept that she does smoke. That's her business, I just don't like when she takes advantage of it, because she is truly a girl who can be stoned all day, 7 days a week. She is better than being a junkie.. And she doesn't smoke when she is with me.

I don't want to be controlling, I don't want to be possessive. I am constantly worried that she would cheat on me.

A guy her age moved into her house and is renting a room. He also smokes weed, and all his guy friends smoke weed, And she has no one up where she lives to
Hangout with, other than these guys who share the same
Interest in marijuana with her. One guy she picked up from one day told her she owed him a blowjob, in a joking way. All she said to him was 'my boyfriend wouldn't like it' (this was over a text). As soon as I found that on her phone, I confronted her, she said he was joking, and that all guys will joke.. And that I had nothing to worry about. I told her I didn't want her to hangout with them anymore. And as far as I know, she hasn't at all.

She often says I'm controlling, and that I am nervous and jealous for no reason. She says she wouldn't ever cheat and that she is a loyal girlfriend. Her mother cheated on her father when she was young, and she knew the man as the 'baby sitter'.. So she says she knows what cheating can do to a family, and that she's better than that.

I find myself really trying to overcome my habits of possessiveness and jealousy. I often imagine her doing acts of infidelity, when she says she's reading or watching a movie. I know it's pathetic of me, and I have no reason to even think this, because I have no evidence of her cheating...

The main concern to me is that I would ruin my relationship with her, and force her to break up with me over my constant nervous rages, where I need the reassurance that she wouldn't cheat. Even mere thoughts of her cheating on me offend her, she can't stand me being this way. She and I have broken up a few times, and every time we do, I see all my flaws and promise to correct them.

She has said its wearing her out. How do I really break the habit of thinking my girlfriend would cheat? I am aware that I am jealous, I don't know what to do. I don't think my girlfriend would cheat on me. But she might if I constantly torment her and provide her with this miserable mindset of mine.

I think she is too good to be true, she's beautiful, skinny, tall, naturally gorgeous.

I hate that I live with this way of thinking.. As soon as I get the courage to displace a thought of her infidelity, I just think something new and bothersome.. It's mentally draining to my girlfriend and I.