How to break plateau/weight creeping up

If you are fit now, and you likely are with doing cardio that whole time, the Polar is underestimating your calorie burn for sure if your BMI is still above healthy range. It assumes bad BMI bad fitness level, which isn't true anymore.
That assumption means low HR means low calorie burn, which isn't true either. The more fit you are (higher VO2max), the more work you can do at a lower HR.

I'd log the weight lifting, it's not that difficult to estimate, the MFP entry for strength training may seem low, but it's actually based on good studies. That's weight lifting with reps and sets and rests of 2-4 min waiting to recharge to hit it hard again.
If you do the circuit training suggested, that is listed under that and much higher calorie burn, which is true.

So you really are eating with a much bigger deficit than you realize.

Playing thread jacker for a moment because i'm really glad to have read this. I have the same hrm as the OP and i have been battling with myself for 3 months not knowing whether the calories burned are accurate or if it is overestimating. For example, this morning, i stepped on my elliptical for 35mins for a low intensity workout, keeping my heart rate between 130-144 and my hrm reported 365kcal burned.

I normally use shapesense.com to calculate my gross calories burned then convert it to net calories burned by using my reported average heart rate which was 137, it said my gross was 299 and my net kcal burned was 253. I also took the 365kcal to estimate my net kcal burned and got 319. Now, using my stats, which one is more likely to be accurate? Sorry if all the numbers have confused you.

I realise there isn't that much difference between both net kcal but when i exercise at a moderate/high intensity level, the difference in kcal is usually higher. It would be great to know this because i want to fuel my body the right way and not let my metabolism suffer.

Female
187cm
27yrs
90.1kg (last confirmed 04.04)

I don't know what you've got your carb/protein/fat percentages set at, but given that you exercise a lot, you might want to up the % of protein. You need enough to make sure your lean muscle has enough to rebuild after exercise (she says like she knows what she's talking about).

I eat a lot of fruits and vegetables (fruit at breakfast, apple and carrots at lunch, banana for a snack, salad and other veggies at dinner). As of now, my goals are set at 50% carbs, 20% protein, and 30% fat, but I typically go over on carbs (from fruit/oatmeal) and over slightly on protein. I have considered increasing my protein intake, but I have found it a little difficult. I'm not vegetarian (as can be seen in my diary), but sometimes I do tend to lean towards vegetarian meals to save some money (I am a grad student). I will try to increase it more.