Although this post is about Montessori parenting I believe many of these struggles are relevant to all parenting and possibly the feelings of inadequacy that many people face. Perhaps you can relate?
1. My first struggle was to think that all I had to do was buy Montessori toys. When Caspar was a toddler I purchased a heap of Montessori toys and I was disappointed when he wasn't instantly engaged. I now appreciate how important it is to observe the child and select toys specific to their interests and developmental stage, to present and demonstrate new toys and materials and to store them where they are freely accessible.
2. Being defensive. I often felt like I had to explain myself and my parenting choices to others. I see this all the time in other parents. Besides your partner or the other parent/s in the child's life, you do not need to explain your parenting to anyone (grandparents included). When someone doesn't agree with me I now accept that I do not need to win them over, I don't need to explain or convince others of how I parent. The less defensive I got about my parenting the more receptive others were to my ideas.
3. Feeling like I couldn't afford to Montessori. Any lifestyle can be expensive. It took me a while to see that Montessori parenting isn't so much about what we have, as how we parent. Our approach to our children is what is most important. Toys and materials are secondary.
4. Feeling like our house isn't designed right or big enough to Montessori. I hear this almost daily from other parents. There are some houses which have a better layout (I prefer open plan) or better features (large windows are always a plus) but it is really a matter of making the most of what we have. Once I got past seeing the faults in our housing situation I was able to see the possibilities that exist. Your child doesn't care if you have a big kitchen or a small one but they do care if they can get a drink by themselves or not.
5. Feeling overwhelmed. Wow this is so easy to do especially when starting out. There are so many ideas out there for activities and how to do and approach things especially in the infant to toddler years. When I first started reading Montessori books I didn't know what do to first and then felt like it was all too much. My advice is to take it slowly, use your enthusiasm to keep momentum for the long term. I also advise not to make over your house or make over specific spaces but make a few changes here and there and then observe your child. You don't want to overwhelm your child with too many changes or doing too much too soon.
While not major struggles there have been times when I have felt like I don't have enough time (especially when having a new baby in the house) or enough patience to be a Montessori parent. I learnt to do the best with the time and patience that I did have. I have also struggled with not knowing if I was doing Montessori 'right'. After a little parenting experience I found that if something felt right then it was right for us!