I think the first thing is to stop feeling like crap! Yes carrying a bit of excess weight may not be the healthiest thing but it's hardly a moral failing or something to feel guilty or ashamed about and your FIL should not have made you feel that way. Having a baby is an enormous deal, both in terms of the strain on your body and the changes to your life, it's hardly surprising that most people put on some weight!
You probably will feel better physically and mentally if you can make some positive changes to your lifestyle but only if these are sustainable, long-term things you positively feel good about doing and are content to make permanent fixtures in your life. Wanting to lose lots of weight immediately is natural but crash dieting is really not the way to go, down that path lies a horrible spiral of misery while dieting then bingeing once off the diet then feeling shamed and disgusting then having to diet again, over and over and worse each time. So I think you need to be realistic that it's going to take you a while to get down to your target rate - a sustainable level of loss is (on average) 1-2lbs a week.
Instead I'd recommend having an honest (but not guilt making!) think about your lifestyle and where you can fit changes in - how do you like to eat, are you a little and often person or do you prefer 2 or 3 square meals a day? Do you need to be able to eat on the run or do you have lots of time/inclination to plan and cook healthy meals? Do you like to eat out a lot? Can you cope with the concept on having occasional junky 'treats' within an overall balanced diet, or do you need to go cold turkey for a while?
There are lots of different plans out there that do work well if stuck to long-term, some examples are low-carb, 5:2, 16:8, weight watchers, slimming world, calorie-counting/My Fitness Pal. There are boards and chat threads about these on the weight loss boards so you can explore a bit and ask questions (lots of very knowledgeable and helpful members). The crucial thing is that whatever plan you pick you need to stick to it long-term (most have modified maintenance plans for when you reach goal weight) so you need to pick something that fits into your lifestyle (so for instance low-carb isn't really for me because I socialise a lot around food and I find it too limiting to have to restrict carbs all the time, I prefer a plan like 5:2 which means you can compensate for heavier eating days with lighter ones), and which still allows you to take pleasure in food and doesn't make you feel miserable and restricted all the time (although being healthy and losing weight does usually mean developing a degree of self-restraint around food and sometimes denying yourself)...
One thing you might be able to do quickly is start to incorporate some exercise into your daily/weekly routine, again it's much better if you can find something you enjoy than feeling like it's 'punishment' or something you have to do to lose weight. Personally I detest the gym and jogging, but I do like brisk walking with headphones in, yoga/pilates, swimming and riding my horses/doing yard work. None of this probably helps that much with weight loss as it's low intensity exercise for the most part, but it does help my general level of fitness and mental wellbeing! I know it might not be easy with a young baby but if you can carve out an hour or 2 a week to do some exercise it might do wonders plus give you a bit of time to focus on yourself. If leaving her is an issue, things like yoga, pilates, aerobics/circuits and HIIT can be done at home from videos/youtube in short bursts in the evening?
Good luck!